All tanking classes have a cooldown which reduces incoming damage by a percentage, and most of us have an ability which restores a chunk of health. Since this is a warrior blog, I am going to talk about shield wall (incoming damage reduced by 40% when glyphed) and last stand (health increased by 30%).
The important thing to notice about shield wall is that you should use it BEFORE you take damage. Suppose you have 30k health. If you use shield wall when you are on 30k health then you can take 30k / 0.6 = 50k damage before dying. If you wait until you have taken heavy damage and are on 10k health then you can only survive another 16.7k before dying. Thus rather than preventing 20k of damage, your shield wall will only protect you for 6.7k.
Following this through, if you don't get a heal, then if you cast shield wall after the first 20k hit, then you will die if you take 36.7k damage before getting a heal. If you had used shield wall pre-emptively then it would have taken 50k to kill you.
Of course if you get a heal then you may be fine, but the point remains that this ability is not something to be used AFTER you take a big hit, but should be used whenever possible BEFORE the damage lands. This means shield wall is most useful if the boss has predictable large increases in damage - frozen blows, fusion punch etc.
The opposite is true of last stand - it is a reliable heal - 9k for our tank with 30k health - and it creates more "room" in your health bar so heals that would have been wasted as overheals now have some effect. It is reasonably powerful for sustained boss damage (because of the overheal effect referred to), but also the key point is that depending on how much damage you have already taken it may be more effective than shield wall - the precise tipping point will depend on your maximum health, but in our example of a tank with 10k health left and 30k max health, last stand will be more powerful at helping him survive until the next heal lands than shield wall.
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