I was watching this video http://www.youtube.com/watch?v=KETNiqHg0oM and it's like nearly every single kill this person gets does the finishing move / throat slit animation, and I barely ever get them at all (I'm at 100 sneak with the shadow warrior perk). Is there some trick to it that I'm not getting? Because I can barely do it once in a while, let alone with nearly every single assassination

I was actually wondering this same thing. I don't know if it's random or not.

Happens to me all the time ONLY in sneak mode. Don't know why it isn't working for you guys.

Perhaps you need higher One-Handed skill?

Ive heard it works more in 3rd person? Idk could be wrong
Maybe a mod?

As long as you're standing right behind them in sneak mode with a dagger it should happen quite often.. At least it does for me.

Power attack maybe? I'm not sure, I don't usually sneak.

I get them pretty much everytime. You must have a max sneak, be perfectly positioned and do a normal attack with only 1 dagger, not a power attack duel strike. This seems to give me an animation every time.

this

Its random chance, you must be on the same elevation, must be close, and it has to be the final killing blow (doesn't matter if its power or not).

It happens most for me if I take one step towards the mark before going for the kill.

Also, be sure you're crouched down in sneak mode.