One of the first talents in the swordsmanship tree is Riposte:
Unlocks the ability to counter an opponent's blows.
I invested a point into it in my first playthrough, but I never really got it to work. I thought it was a passive effect of blocking, but that doesn't seem to be the case. It seems to be an active ability, but I haven't found out how to actually activate it.
How do I use riposte and counter-attack? Does it use up vigor?