Pithing and decapitation are not the same. Pithing is quite literally destroying the brane case. I have only ever done this once and hope never to do so again! In my case a wild bird but it is the same. I used a pocket knife it is what I had. The blade is pushed through the top of the skull, and then quickly moved side to side (while holding the skull still) this scrambles the brain. I have seen it done by crushing the skull. Reptiles have strong bones so hesitation is very bad. It is a fast and hard action more force is better than less force.