If you are lookin' for a decap aim for the neck area, and hit the neck with your hand, or foot which are the easiest to decap. But make sure you build up speed with the joint you want to hit the neck with, you can do this by moving the abs, rotating the chest, or extending ankles to jump higher etc. So strike the neck with a powerful move and you will always decap, and if you don't get it right the first time you can always press "R" to play your replay again, and press "E" where you think you can improve and if you want to see both ghosts to see how strong or hard your hitting press "B" to see both Ghosts.
Hope this helps.