If you slap the head right from above so it presses downwards will most often break the neck joint easily, so any blow coming from above, or below (skimming through the back) will most often remove the head, also a direct blow to the neck does this.

For a simple decap, do the following;
85 distance
500/10

Hold all
Contract pecs
Raise shoulders
Space
Contract hips
Contract abs
Lower shoulders
Space(for the landing, continue here)
Space
Extend hips
Extend ankles
Extend elbows
Extend wrists
Raise shoulders
9x space
Extend abs
Contract hips
Lower shoulders
Space x2
Extend glutes
Extend hips
Raise shoulders
Extend pecs
Contract ankles
Space x10(now for the pose)
tap C so all are stiffened again
Contract ankles, knees, hips and glutes
Extend abs
Space X3
Extend neck
Extend ankles
Extend glutes
Contract abs
Extend pecs, lower shoulders (yes, i'm aware you'll land on your head, but nicely packed up)