Let me start this guide by saying that a tank is not a necessity in City of Heroes, and that is one of the things that makes this game what it is. The support teammates have the potential to turn any content in this game into trivial encounters. Even the least survivable archetype in this game can be built (with very much effort, mind you) to have enough survivability to withstand countless enemies. The need for a tank shows up when you have fragile teammates and less than the optimal kinds of support. Where a good tank will shine the most is on a terribly fragile team. He is what holds the team together and turns the game into a war of attrition that the tank will not lose.
When it comes to tanks in City of Heroes, I have only ever seen two kinds. There are the ones that excel and the ones that are a wasted slot on the team. It really is that simple, because if the tank is not doing what he should be doing, then anyone else could do everything that he is doing, better.
You want to be the kind that excels. I know that because you are sitting here reading this. Well, either that or you are a blaster that wants to understand what the good tanks really are doing for you. Regardless of your motivation, I encourage you to read on. Knowledge can only make you a better player. Of course, before I teach you the Last Rule of Tanking, I have to get through a few others.
The First Rule of Tanking
The tank is a role, not an archetype.
Do not think that just because you select the Tank archetype that you are immediately bestowed with all the ability that is required to actually tank. Further, the Tank archetype is not the only source of tanks in this game. A Brute may be a tank and a Tank may be a melee DPS (albeit a relatively poor one). Again, this is one of the strengths of this game; no one is shoehorned. It is true that some archetypes are naturally suited to fit the role of a tank, but keep in mind that anyone can tank. As content becomes more difficult, of course, the selection thins and only those small few archetypes designed for it can consider themselves true tanks.
This guide is going to focus mostly on tactics that work for melee oriented tanks, but I will have sections dedicated to the unconventional tank.
The Second Rule of Tanking
The tank sets the pace of the team.
There is a motto among tanks, it starts with "First to fight," and I will get to the rest of it later. The motto starts this way because one of the tank's main responsibilities is to be the initiator. Every fight has to start with somebody to engage it, and as the tank, you are that somebody. I am not saying the tank has to be the leader of the team, but he does have to be the first one into the fray.
As the initiator, it is your responsibility to know when the team is ready to handle the next challenge. The enemies show up in groups, allowing for breaks between fights if needed. Starting combat too soon could mean that some players are out of endurance, low on health, or waiting for key powers to recharge, leading to a much tougher fight. A few seconds to recoup could mean a difference of minutes in the fight. Most people do not want to waste time, however, so you have to find the point where recovery time does not become idle time.
Another job that goes along with the "First to fight" rule is gathering the enemies into a tight group. Most of your teammates should have abilities that hit foes in an area. The closer you get the enemy group before your team gets to them, the easier it is for their powers to hit the maximum amount of enemies.
The final point to take away from here is that an initiator requires mobility. More than just "getting there first," not all enemies are static. This game does have ambushes and patrols, things that could easily reach the members of your team that are prone to go squish. Mobility is what allows you to intercept these groups and initiate these fights as well as the static encounters. So get that Stone Armor Tank of yours some speed bonuses or Teleport to get around.
The Most Important Rule of Tanking
The tank who is the last man standing, has failed.
Please, if you take nothing else from this guide, learn this rule. Live by this rule. The ultimate purpose of a tank is to protect his team. No one comes to harm on your watch. To continue the motto from before, it becomes, "First to fight, first to fall."
The reason a tank initiates combat is because that first volley of attacks, often called the "alpha strike," is the most dangerous. That is certainly not the only time teammates can be hurt, of course, so you need to do more than just stick your neck out there. Before I can tell you how to protect the team, I need to go over exactly how enemy aggression, or "aggro," is handled by the game.
How Aggro Works
Every enemy has to decide which player to attack. It does this by assessing a "threat" value that is calculated for each player the enemy is aware of. These values are then sorted into a list and the enemy attacks the player with the highest threat on this list that the enemy is able to.
The formula, as close as the game developers will tell us, takes into account the amount of damage a player has done, the range that damage came from (melee or ranged), the archetype of that player, any secondary effects from that player, and a taunt value. It is important to note that this taunt value is dependent on the taunt duration which is remaining. A fifteen second taunt is very strong when it is first applied and much weaker thirteen seconds later. There is another factor which is unique to the type of enemy which may cause it to be threatened by some attributes more than others, but any more details of this has not been released.
There is another aggro list you need to be aware of as well, your own. At any one point in time, you can have no more than seventeen enemies attacking you. This aggro cap was put in place to prevent zone wide "herding" of enemies. This list is sorted by only two factors. The enemies that have been most recently attacked are at the top of the list. The second sorting mechanic is enemy rank, with higher rank enemies given priority. If you could somehow attack seventeen of each bosses, lieutenants and minions all at the same time, only the bosses would retaliate. What this means is that, even if you could be the highest threat for any more than seventeen enemies, the enemies at the bottom of your aggro list will be forced to attack someone else.
How to Control Aggro
Controlling aggro comes straight from understanding aggro. The first thing to do is generate the threat needed to keep enemy attention, which means maximizing your personal threat value that gets calculated. The threat formula takes into account your archetype, which is one of the reasons that Tank and Brute archetypes have an easy time filling the tank role. Their threat modifier value is four; dwarfed Kheldians and Scrappers have a value of three, while everything else is two or less.
The factor which most commonly determines who gets attacked, and has the strongest effect by far, is taunt and its associated duration. Again, this is a reason Tank and Brute archetypes make the best tanks. Many of their powers have a taunt value baked into them. Any Tank single target attack which lands will taunt five enemies nearby. A Brute's attack is similar, but only taunts the enemy hit. Most melee defensive sets have an aura power of some sort, be it some sort of damage, debuff, or simply a taunt effect. On a Tank or Brute, these auras always taunt. For a Scrapper, the damage auras do not taunt. Lastly are the powers Taunt, Confront, Antagonize and Provoke. Tanks and Brutes have access to Taunt, an auto hitting taunt power that hits five enemies. Scrappers have access to Confront, which is identical to Taunt except it only hits one enemy and for a shorter duration. Antagonize is only available to Kheldians while in dwarf form; it acts just like Taunt. Provoke is a pool power available to anyone; it is similar to Taunt, but it can miss and has the shortest duration.
The most commonly overlooked factor is actually damage. I have seen far too many "tanks" who believe they can sit back and repeatedly use Taunt as their sole means to maintaining aggro. Equally egregious is the "tank" who does not slot his attacks well enough to use effectively. All of the effects in the threat formula are multiplicative; lacking damage will make your job very difficult. The Blaster may have a low threat modifier, but when he starts dishing sugar coated nukes out to the arch villain, that is going to garner some attention. Most attacks have secondary effects attached to them as well, just one more thing to throw into your threat formula. Even more than all of this, teams can only be successful with damage. You must pull your weight on a team by contributing to the end goal: defeating enemies.
After you have a handle on generating threat, the next step is keeping the correct aggro. In situations where you can keep the entire enemy group's attention, you need to rotate through them as you attack. This is imperative for tanking Brutes because it is the only way to spread their inherent taunt through single target attacks. Area of effect attacks are always preferable, as you keep more enemies aware of you per attack. When the groups are larger than your aggro cap allows, remember that your primary job is to keep the team safe, so you want the meanest and ugliest enemies to attack you. This typically means the bosses, which is fortunate because they get the rank priority to attack you anyway. There are cases, though, where lieutenants and even minions can be particularly deadly to your teammates. If it is a particular minion (e.g. Malta Sapper), be sure that you are hitting him with a single target attack occasionally to keep him from falling off your aggro cap list. You may also have an attack of some sort that offers control (e.g. Freezing Touch or Char). Enemies that do not attack at all are better than enemies who attack only you.
Finally, you need to keep the positioning of your enemies, your team, and yourself in mind. Most enemies are not limited to single target attacks; just because they are focused on you does not always mean your team is safe. There are many foes, and a few arch villains in particular, with strong cone shaped attacks; remember that the enemy attacking you is also facing you. Use this to turn him away from your teammates. When the enemies throw very large fireballs, do not stand near your squishy teammates. Some team members will want to be in melee range which is typically near you, though presumably, they know what they are doing.
I lied. There is one more point to controlling aggro and it ties back into pacing the team; you need to know when you do not need to control aggro any longer. Some teams are strong enough to essentially "mop up" right after you take the alpha strike and clump the enemies together. A few teams may need you to stay at each group until every last enemy is dead. Obviously most teams will be somewhere in between. You will need to determine what kind of team you are working with every time the team changes and act accordingly.
Dealing with Large Over-Aggro
What I mean by large over-aggro is an extra group of enemies, far beyond your aggro cap of seventeen. The most common cause of this is an ambush or a patrol. It also happens often when enemy groups are very close together. Every once in a while, you get an ambitious team member who comes running back to you with his own aggro cap of angry foes.
When this happens you have to remember that, just because you cannot have the attention of more than seventeen foes, it does not mean you cannot control more than seventeen foes. Mobility plays a huge role here since the groups tend to come from different directions, as do any control powers you may have. If nothing else, be sure to tag every boss you can find. Jump back and forth between groups, use your weaker attacks and Taunt on the bosses, and try to finish off the minions with your stronger attacks. If you have something like Footstomp or Salt Crystals, use that to control one group then jump back to the other. Any location effect powers, such as Quicksand or Caltrops, are great ways to intercept an incoming ambush. If nothing else, it will buy you time to deal with the foes in front of you before the ambush gets to you. Attacks like Shockwave and Repulsing Torrent usually get skipped because of their knockback, but the mitigation of knocking foes away from your teammates is invaluable in a situation like this.
Evaluating Taunt Auras
Taunt auras have attributes just like any power, and all taunt auras were not created equal. The first important value is the taunt magnitude and duration. Second is the pulse rate, which is how often it applies its taunt and other effects. The "other effects" can include a wide range of debuffs and damage. Generally, the auras with the longest duration are most powerful. The pulse rate has a smaller effect on power because, in all but one case, the taunt from an aura will not stack with itself; it just refreshes.
The strongest is most likely Against All Odds from Shield Defense. It pulses once every second, is tied for the longest duration (17 seconds) and includes a debuff. To top it all off, the power buffs your damage, only increasing the threat you generate by your attacks.
Ice Armor is a close second and a special case because it actually has two auras. Both have the same duration (13.6 seconds) and the taunt values do not stack with each other. One is the standard damage aura that pulses every two seconds. The other, Chilling Embrace, is very unique; it pulses twice per second and has heaps of guaranteed debuffs for speed, recharge and damage.
Most of the rest are pretty even with each other. Damage auras pulse every two seconds, the others pulse once per second.
The last case to cover is Rise to the Challenge from Willpower, which is another unique beast like Ice Armor, except in a very bad way. Every other taunt aura has a magnitude of four, this one has a magnitude of three. The real problem is the duration of 1.25 seconds, less than one tenth the duration of the other auras! Keep in mind that duration remaining is part of the threat formula, so this directly affects Willpower's ability to gain and keep aggro. The saving grace here is that Rise to the Challenge, unlike all other taunt auras, can stack with itself, but not without enhancement. Once you throw some taunt enhancement in here, it can stack up to a magnitude six taunt effect with over a two second duration. The extra magnitude helps to make up for the lack of duration, but you will have to lean more heavily on Gauntlet/Fury and the power Taunt.
Scrapper Specific Aggro Control
Scrappers will have a difficult time filling the tank role because they do not have the inherent tools to do so. Scrapper attacks do not generate taunt in any way, and the majority of defensive powersets have the taunt removed from their auras. I am not saying it is impossible, mind you, just difficult. If you intend to tank with a scrapper, you will do best to select Invulnerability, Shield Defense, or Willpower as your secondary, as each of these includes a taunt aura. The taunt durations do not change from Tank/Brute values, but all the magnitudes become three. Leveraging this with your higher base damage, you should be able to keep aggro away from squishy members of the team. Do not expect to take the aggro off of a Tank.
Specific Aggro Control Tactics
Here I will be getting into some neat tactics or special tricks that you can employ while tanking. Not all of these work in every situation, of course, but I will tell you some general guidelines about when to use them.
Ranged/Melee AI Mode
Enemies in this game have a preference to either fight in melee range or to stay back and shoot all day. Most of the time, you want them in melee range because they will be closer together and the team's area of effect powers will be more useful. It is also easier to control aggro that you do not have to run after. So how to you get them to punch you on the chin instead of shoot you in the knee?
Taunt, if you have it. It carries a -75% range debuff, which will force enemies to close in on you. If you do not have taunt, there is another option. Enemy AI tends to get stuck in one mode, and if you engage them in melee range, they will often retaliate with a melee attack that also puts them in their melee mode. As you walk away from this enemy to find another, he should not revert to ranged mode and continue to follow you around.
Break Line of Sight
This is a tactic that will help you bring enemies closer together when they are not as easy to put into melee mode. An enemy who wishes to hurt you must have a clear "line of sight" to you. This means that despite the range of an attack, it cannot fire around corners or through solid objects. If you put environmental objects between your enemies and yourself, they will be forced to close most of the distance until they regain a clear shot on you. Typically, they all do this at the same spot, which conveniently clumps them into pleasant little area of effect begging bundles.
Again, some enemies will refuse to move from their position when you want them to get huddled up for a group burial discount. One way to make them move is to follow dad's old line and put a boot up their collective rear end. This should be common sense, but so many people have a fear of knockback effects that I find this worth mentioning. The fear is undeserved; knockback itself is not the problem, improper use is. If anyone on the team, the tank should have some kind of knockback power to position enemies.
Not every team can handle every group of enemies. Whether the arch villain's mob is a little too tough for your team as it stands or two groups are too close together to safely engage, there is an effective way to thin the crowd. This is typically called "pulling" the enemy, because you essentially pull him away from his group. This is most typically done by standing outside of the range which an enemy will see you and using a ranged attack to strike one enemy. Line of sight is then broken so that the enemy must close distance before retaliation can be made.
This is not as easy as it sounds and there are quite a few unseen mechanics at play here. When you strike an enemy and notify him of your presence, there is a chance that enemies nearby will also be notified. The greater the damage of the attack, the larger this chance will be. Further, higher ranked enemies have a higher chance of notifying their own minions to the attack. Another factor is enemy perception or your stealth value. The less visible you are, the less likely that additional foes will be alerted. Finally, proximity to other enemies matters. An enemy in the middle of a group will more likely bring friends than a foe on the fringe.
Putting all of this together, try to have some stealth and use a weak single target attack on a minion that is farthest out from his group.
The Jump Taunt (or Attack)
This maneuver is one that is almost always a good idea. It requires some air mobility, and for that I recommend the power Combat Jumping. The idea is simple, run towards your foes with an enemy at the front of the group targeted. Begin your jump and activate Taunt or some area attack. Taunt can be activated at any time after the jump begins; the area attack, if it is a point blank effect, will need to be activated close to the enemies. Momentum will carry you past the enemies targeted, landing you behind the enemy group.
At this point, you have already obtained a solid hold on the attention of the front line enemies, while turning them to face away from your teammates. You have also put yourself in an ideal location to control the aggro of the rest of the enemy group. Quick and easy.
Targeting a Teammate
This tactic is most commonly useful in over-aggro scenarios, but it can be used any time one teammate starts to take a righteous beating. Uncontrolled aggro can be a viscous thing because these enemies tend to head straight for high damage dealing teammates, particularly your glass cannon blasters. The saving grace here is that, generally, the endangered teammate is already targeting whatever is trying to kill him for two reasons. The first being that aggro is hard to gain without first hitting something and secondly, when that something starts to beat on you, the natural reaction is typically kill it with FIRE!
With any luck, he is taking damage slow enough that you can react to the crisis. One of the quickest ways to react is to target that teammate and activate Taunt or any attacks you can. The attacks will be redirected to whoever that teammate is targeting. Additionally, you can press your F key (default "follow" command) to run towards the teammate without having to spend time finding him.
Largely an outdated and time costly technique, herding can be used to greatly reduce the risk your teammates face. Herding amounts to the tank going off on his own to grab the full aggro cap of enemies and herd them back to a location. When the enemy group is sufficiently piled up on top of one another, the team appears from behind their cover and unloads a full arsenal of attacks. The largest source of inefficiency here is that on a full team, the tank cannot herd more than one group of enemies before he is over his aggro cap. At that point, the vast majority of teams are better dealing with each group where it stands.
Where herding can boast great performance is on smaller teams when enemy groups are likewise smaller. The tank can enter a room and pull three or four groups together to maximize the damage dealt from area of effect powers. Still, I am not convinced this is the best tactic for this situation.
Unlike the previous tactic, this one is not detrimental to team efficiency at all, so long as the team is strong enough to handle a little bit of over-aggro or can kill things quickly. Herding forward refers to grabbing the aggro of an enemy group and then moving forward to the next group. On a full team, this will create an over-aggro condition, but it essentially allows the team to kill two groups at once. On smaller teams, this becomes less risky because jumping to the next group will not create over-aggro.
The Unconventional Tank
Tanks come in all shapes and sizes and not all of them are melee archetypes. There are quite a few unconventional tanks that can still fill the role in most content. Defenders, controllers, dominators, corruptors, masterminds, and even blasters have tanked for teams under the right circumstances. Generally, these kind of tanks have a very difficult time actually holding the aggro of the enemies. Most of this is handled by being the initial contact with the enemy. This carries a fair amount of weight in the AI scripts, because you need to actually exceed the threat of a teammate by a margin before the enemy will actually switch targets. It also means you need to be able to survive that initial alpha strike.
Holding the aggro is not always necessary. Remember that the main role of a tank is to protect his team. To that end, there are a few ways to "tank" for your team without having solid aggro control. I will detail a few tactics for the unconventional tank, but keep in mind that most of these are intended to be used in tandem with one of the others.
Tank with Control
Controllers and Dominators make the best use of this method, but I have seen even Blasters do this to some extent. Using powerful area control will keep most of the enemies from attacking. Bosses become difficult, because without a fortuitous Overpower or Domination active, they require a second application of control in most cases. Some enemies are resistant to certain controls as well, and every foe is subject to the unfortunate miss.
Do not forget that hard control is not the only kind. Knockback can be used to great effect on a fairly large scale. Anyone can take Air Superiority as a tool to juggle boss rank baddies. In addition to the entirety of Energy Blast, there are area knockdowns like Ice Patch and Earthquake that can keep foes off their toes. Environment can be used to magnify the effect of these powers by limiting your enemy's access to you. Fear powers are usually quite far reaching and will allow you to take the enemies at your own pace. Endurance draining is typically difficult to maintain, as you need to keep the -recovery active as well, but when done right it is as good as the hard controls.
Tank with Pets
This is generally the least reliable method, as pets do not keep aggro very well at all. The largest advantage is that, generally, no one cares that their pets die, so it is perfectly acceptable to sacrifice them for the good of the team. Illusion Controllers are notorious for their ability to entertain foes with their (literally) indestructible Phantom Army, but they are far from the only pet centric tank. Other controller pets can be used similarly, though they are not as easily put into the fray and certainly not as survivable. Masterminds can order their henchmen in first (although I believe they benefit more from being the tank themselves). Thanks to the Patron Pools, almost anyone can have a summonable pet for the occasions they need a pet tank for a brief time.
Tank with Debuffs
Debuffs can be used, like controls, to keep the entire team safe. Most commonly are -tohit and -damage debuffs which keep enemies from landing attacks or making the attacks much weaker, but not to be forgotten is -recharge, which is easy to stack to its limit and keep enemies from attacking often. One of the advantages to tanking with debuffs is that they help you keep the aggro on yourself for a bit more control of it. Some debuffs are anchored on a certain foe, in this case it is usually best to put it on a boss so the anchor lasts longer. You will also have to keep positioning in mind to ensure most of the enemies are affected by your debuffs. Anchored and most location debuffs benefit from being auto hit, as well.
Tank with Buffs
Similar to the previous method, this keeps the team protected from the enemy attacks, although without the extra aggro control a debuff entails. The advantage here is that buffs do not "miss" teammates. Generally, instead of controlling the enemy aggro, your focus is on controlling your teammates to keep them within range of your buffs.
Tank with Damage
Dead foes harm no teammates. Unless they are Nemesis, but that is another story. The point is, if you can kill the enemies quick enough, you have essentially protected your team. This is not a good tactic to rely on for every spawn, as it usually means pulling all the stops and using your nuke, meaning you will have a long time to wait before you engage the next spawn with the same tactic. Use your damage boosting powers and front load as much ouch inducing power as you can muster. When the dust settles, hopefully you and your team will be the only ones standing.
How to "Control" Teammates
There is an unseen foe that runs rampant in this game that cannot be damaged, cannot be debuffed, cannot be taunted and truly cannot even be targeted. The foe is known to many as the incurable "stupid." If there is one thing I have learned from all my time tanking, it is that you cannot save people from themselves.
You can, however, make it harder for them to hang themselves. The most common problem is an overactive squishy teammate racing to fresh groups of enemies because he enjoys watching things burn (himself included, sometimes). Most of this will come down to the second rule and beating the fool to each group of enemies. You may also use the same tactics as dealing with large over-aggro, because the fool is likely causing those conditions anyway.
No matter how stupid, though, you cannot sacrifice the safety of the others on the team to save one. At some point, you have to wash your hands of the situation and pray he learns the hard way.
Be careful that you do not assume anyone who runs off on his own is an idiot. There are many characters which are more than capable of soloing entire groups of high level foes. These capabilities are not limited to a subset of archetypes either. Use your judgment. If the loner is not endangering himself or the team, it is probably best to let him be.
There is an additional case that may appear stupid at first glance: the melee squishy. Good examples of this are melee centric Blasters and Dominators, or most anyone with the kinetics powerset. These guys have specifically tooled their build to make the most use of the high impact melee ranged powers. They offer significantly increased potential, but the increased risk to survival is significant. A good tank can make their survival a great deal easier, so long as he recognizes what is going on. I touched on this earlier; keep your positioning in mind to keep AoE powers targeting you from hitting these squishy team members. You should also keep in mind that these teammates will have a larger chance of drawing aggro off of you due to their range and increased damage. It makes your job a little more difficult, but the reward is well worth it.
The Last Rule of Tanking
You must survive.
We all knew this was coming, what with the jumping in head first and the taking damage for the rest of the team. It is time we finished the motto. "First to fight, first to fall. Never fall."
But why, you ask, is this the last rule to be taught? I call this the Last Rule of Tanking because it is the least important. You can have all the survivability in the world and it does naught for your team if you do not take the hits for them. In fact, I would go so far as to say survival is optional. If I could revise the motto, I would make it thus, "First to fight, first to fall. Never stay down." That is just not as catchy, though.
Most tanks will lose their control of aggro if they die, and for them, the first motto works. For a select few, i.e. Fiery Aura and Dark Armor, they only need to survive long enough. Both of these powersets have an offensive self revive power which stuns the nearby enemies. I have said it before in this guide, enemies that are not attacking at all are better than enemies that are attacking you.
That being said, survival still makes any tank's job easier, and I will provide some general advice on how to increase your survivability.
How to Survive
I do not intend to detail how to build each powerset for survivability. Most of that is straightforward enough; true survival, however, is much more than getting accolades and building resistance, defense and regeneration. What I will cover are the less than obvious methods to hold out against those rough encounters.
First you need to find out what your weaknesses are, and do not fool yourself by thinking you have none. Every set has an Achilles Heel and you need to know how to cover for it because you cannot just tell your team what they have to avoid. Keeping the right inspirations around is the easiest answer (but it is certainly not my answer). You can also craft temporary powers and empowerment buffs from your supergroup. Do not be afraid to ask for help from the team, either. It is less embarrassing to ask for extra Luck inspirations or specific buffs than it is to ask for an Awaken or a revive.
There are some tactics you can use as well that are particularly useful against hard hitting arch villains. Learn to kite your enemies by keeping mobile and jousting with your attacks. Melee attacks usually hit hardest, so this method restricts your foe to using his ranged attacks. Likewise, you can find ways to stay out of range, either by flying above the foe or letting your team immobilize him.
The Role of the Off-Tank
Fairly often, you will be on teams with more than one tank. It these situations, it is important to designate one player to be the primary tank, or main-tank. The other tank then becomes an off-tank. The main-tank effectively takes the lead and the off-tank will serve as a sort of backup to everything the primary does. If there are three or more tanks, there is still only one main-tank. All the rest are off-tanks.
In practice, there tends to be very little difference between what the main-tank does and what an off-tank does. Regardless, the reason to designate roles is to make things easy for the team. The team stays much better protected if the tanks know who is supposed to soak which kind of aggro and do not end up fighting for the attention of the same mobs. Further, no one will raise the question of "which tank do I follow," and it helps to avoid conflict between tanks. Nothing in the game can defeat a team as well as arguing players.
When deciding who should be the main tank, it will most often come down to which player is more comfortable with the role given the task ahead. This decision is not final, it can change between missions and even mid-mission as well, especially when the enemy groups you face or the team composition changes. Assuming players are equal, though, the off-tank should be the more mobile of the two. If that is likewise equal, then the tank with the highest damage potential should be the off-tank.
You will run into a few cases where one player insists on being "The Tank" (or some other variation implying imagined superiority) and in these cases, it is best to just let him have it. Remember, one of the major reasons to designate in the first place is to avoid tension and conflict within the team. As an off-tank, you will likely do just as much to protect the team as "The Tank" will.
Off-Tanking the Normal Stuff
Most of the time, while off-tanking, you will play like someone in a melee DPS role that just happens to have one eye on his teammates. Your primary concern will be defeating enemies, which is why the higher damaging tank should have priority when choosing roles. It allows you to focus your attention on damage dealing powers, even switch to a more damage oriented build, because the need to lock down your aggro control will be significantly diminished.
Your next concern will be taking care of the stray aggro, anything the main-tank missed, as well as any over-aggro that happens. Once you get the attention of the extras, you should move to the opposite side of the main-tank from them. This aids the main-tank in controlling all the aggro by forcing the enemies past his taunt aura and attacks, as well as collapses the strays into the thickest group of enemies for your team to dispense with area attacks. These movements, coupled with the nature of over-aggro (ambushes and patrols), should make it clear why an off-tank should have the better mobility.
On faster teams where you feel comfortable enough, it becomes beneficial to play a game of leap frog with the main-tank. What I mean is to jump ahead of the team and get the next group of enemies compacted on you. When they finish the group they are fighting, the main-tank will run ahead of you and do the same thing while the team quickly burns down the group you have clustered. The team will bounce between tanks, always having a groups of already bunched up foes to defeat efficiently.
Occasionally, you will be on a large team that can operate faster as two small teams. Most often, this will be in cave type maps with multiple paths. Essentially, you take half the team with you and start playing the main-tank for your members while everyone else goes a different way. As soon as you rejoin the other group, though, be sure to revert to your off-tank mentality.
Off-Tanking the Hard Stuff
Again, this is much like being a main-tank, except that you focus more on just killing the big bad instead of keeping his attention. There are still a few cases worth detailing.
There are a few arch villains that trigger ambushes as you fight them. In these situations, it is the off-tank's responsibility to intercept those ambushes, allowing the main-tank to only worry about surviving and positioning the AV. Sometimes the ambush is another AV, in which case you have to work out who stays with which tank. You can also bring that ambush aggro back to the main-tank, provided he can survive all the attention.
Some arch villains cheat. They have hundred magnitude holds or stuns, or they just hit really really hard. Sometimes they just get lucky and the main-tank has trouble surviving. In these cases, the off-tank should be prepared to take the ugly aggro for a while. By playing a game of hot potato with the AV, effectively tossing him back and forth as you get hit, you can greatly extend your tanks' lifespans.
Some arch villains only come in groups, most notably in the Statesman Task Force and Lord Recluse Strike Force. These missions will pit your team against a large group of very powerful AVs, and the one of the best ways for a tank to survive is to share the aggro of the group. Another example is in the Imperious Task Force, where the Nictus Essence AVs can be drawn away from Romulus with relatively little danger, protecting the team from the auto hit damage of one nictus and denying Romulus his heal from the other.
That is everything I can think of. Now go protect your team.