It seems like they're suppose to be dramatic and in slow motion, but a lot of the time the animation plays out at hyper speed and my character quickly jerks back into the default pose. Is this just how they are?

Also, what initiates a finisher?

Some of the finishers are a little rushed looking. But I think they all look amazing.

Usually a finisher occurs when you kill the final enemy of a group. Sometimes they'll trigger if you are fighting a single enemy with no one else around.