After powerattacking many stormcloak soldiers (30-40) i never got a special move.
I find that power attacking doesn't always work. Final kill animations, to my knowledge, only happen under certain conditions.
- The victim is not running away.
- You are facing the victim, unless it's a backstab.
- There are no other enemies around.
- You kill him in a single shot. (It can't be slice slice slice, his health has to be low enough to kill him with a single click instead of continuous strikes)
- He is not downed.
So, to me, I kill every body else first, lower his health to a certain point, leave him, return, face him and strike. That almost always gives me a finishing kill.