This is the first legitimate decapitation I've ever done, which is why it surprised me that it wound up decapitating both fighters.

Distance must be set to 200 for this to work.

Contract both knees
Space x1
Hold all
Extend ankles
Space x8
Extend knees
Contract hips
Raise shoulders
Contract pecs
Grabby hands
Space x4
Lower shoulders
Play (Press P)

Enjoy.