I should add one more point for clarification, a point that has been made by several scholars, including Dr. Mantik: JFK's backward movement consists of two movement--first the backward movement of his head, and then, logically enough, the backward movement of his shoulders and upper body. Itek discussed this fact in their study. Dr. Mantik says that we simply do not know how much the editors exaggerated the speed of these movements but that the original movements were certainly markedly less rapid.
Way back in 1998, based on Dr. Mantik's and others' research, I said that the backward head snap was actually evidence of alteration because the movement was too rapid to have occurred in the real world because it could have been caused by a bullet, a muscle spasm, or a jet effect.