So there has been only once or twice that this has happened in my hundreds of hours of play the game, but there are kill move when your enemy is on his knees. I've worked at it, but ive never been able to replicate it. The two Ive seen are, Your character will kind of scoop the enemy with his foot so he is on his back, slightly propped up, and shove the sword through his chest all ceremonial sacrifice style. The other one is when he does some kind of upward swing and decapitates the enemy. They both happened when I was standing still in front of them and did a standing power attack. So why would bethesda add a set of kill moves that have such a rare occurrence chance?

They're all random. I get the first one you said all the time. And the second one you said will only happen if you have the right perk.

But while they are on their knees? I only get executions if the enemy is up and about, and I always get a cam if he is on his feet, and he is the last one. But I never get it if they've gone down.

Yeah, there's couple of killcams on downed foes. Striking from behind sometimes does the impale move. Two-handers have an executioner-style decap! o_o'

I've gotten one with one-hander, where you drive the sword through the top of their neck from above... ick.

I get one all the time while dual wielding. I swing one sword at his leg to make him drop to his knees. Then I cross my swords like a pair of scissors with his head in the middle before I cut it off. I've seen this kill move at least 100 times

There's one where you're dual wielding when they're kneeling down where you slowly position both of the blades on their neck in an X shape and then quickly decapitate them.

http://www.youtube.com/watch?v=xM-claQ7yiA

It's done a few times in this video, at 0:16 and 0:24 with the conjured swords. That's one of my favorite kill moves

Both of those were done while the enemy still had a decent amount of health and was still up. I've kinda figured it out. When you are fighting enemies, and they reach about a 16th of their health, 1 of 2 things happen. 1 if you are standing in the right position and they are the last enemy currently engaging you, they will be finished with a cinematic kill move. 2, when there are still enemies left or the angle doesn't work, they will fall to their knees and begin crawling about. I'm saying executions happen very very rarely when they are in this crawling phase, and I want to know whats up.

I find that if you just play and don't TRY to get animations, you'll get animations.

I have no problem getting animations. I have problems getting animations when enemies are in the downed state.