Everytime I stealth killed somebody, it was just the usual slash
But how do you trigger the special finishing moves?
Or do you need a perk in stealth?

Good question. I unlocked the entire stealth tree. Never get to execute such moves.

i used a cut scene kill is that what you are talking about? if so it seems to be random

Sneak, get behind them, use a power attack

Yep, the power attack triggers the finishing moves in stealth or not.

I think it depends on a few things but Power strikes seems to trigger more cutscreen kills.