How does the barbarian "revenge" skill work?

It's a skill that doesn't work with the usual cast and cooldown mechanic as other skills do. Instead it will activate in 15% of you getting hit by a monster. You'll see that the skill-icon will turn active, but then you'll have to manually activate the skill.

The time-window for activating the skill is also limited. After some time the skill will become inactive again if you didn't chose to activate it.

While it's different from other skills, I personally like the mechanic and the skill already helped me in several situations (surrounded by monsters, this skill can be some sort of "panic-button" that heals you and deals damage in the same turn).

Update: Also note that the skill doesn't have a cooldown. So if you're getting hit a lot, you can potentially trigger this skill quite frequently. Especially if you enable the "Provocation" skill-rune (available at level 52) which increases the chance of activation from 15% to 30%.