I recently unlocked the Barbarian "Might" skill called "Revenge", but it confuses me. It states that on hit it has a 15% chance of becoming active, but it doesn't seem to ever deal damage to enemies when I'm getting hit. I'm also a little confused about having what appears to be a passive ability in a skill slot, since I thought the passives were only at the bottom of the skill menu. I've tried mashing the button during the fights fight a few times, and I just see the red text "this skill is not ready" over an over at the top of the screen.
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%.