Are these just random occurances, or is there a trick to them? IT looks badass when you're chasing down a mage, grab them by the head and shove a sword through their back, etc. Also, the dragon kill animations are sweet.
I know when a enemy falls to there knees there is a higher chance of getting one. One of my favorites is grabbing someone by there hair and bashing their face in (unarmed).
I love the one-swing decapitation moves. Makes me feel like a real warrior. Also, they are more prone to occur when you attack an enemy with really low health (how it usually happens with dragons). I guess the actual scene depends on the angle or pure randomness.