Go the opposite way he is going

You not need that much momentum. You just need to "cock" it by turning the opposite direction then release and pull it to the other side of the "cock". Try to move as much joints that aid the turn. Attached is a replay of something like that^.

Aim the joints with enough speed to break a joint.
check out my WIP of a madman.
Its all about frames and aiming. To aim a joint, simply do your move, then keep skipping 1, by 1 frames with Shift+Space, because 10 is too much for a decap.

try to use a limb to bend the head far back enough to snap it off while pulling it
like using a knee to add pressure to the neck for the decap.

Mate i tried to decap spinnic my body left and right but i won't work, try punching/kicking his head off

The ripping of the head can only be done by certain motions before you get to the head of the oponnent, and sometimes your move wont work all the time based on the oponnents move.