Wow, I hadn’t heard that theory.
Can’t recall where I heard it...
Though now that I am looking again it does appear that his head snaps down before going back... That is what a head would do if it was hit with a downward momentum externally... not sure what physics would happen if shot was up through neck and out through the head... seems it should be just snap backward... and not go down first... but again not an expert...
Momentum entering body at the neck, then out of the top of the skull, I don’t think there would be enough time for the upward momentum being absorbed to cause the hesd to snap down before the exit would snap it back.. but someone with much better understanding of the physics can answer that.