Interesting. That is the one item that has always lead me to accept that another gunman was involved. I don't understand how it works that a head would move backwards when shot from the back .. unless the car accelerated at exactly that point in time. I will check it out. Thanks.
The entry is very narrow and actually imparts relatively little direct force. As bullet traverses the skull it dumps energy/pressure into the contents. The exit blows a larger hole where the now pressurized brain matter exits like a rocket, forcing the head toward the shooter.
Could it be that the head ‘appears’ to be reacting to the forward movement of the car? I don’t believe/recall that the car was stopped and as such all factors of movement eg. car, Kennedy’s own, the impact of the bullet, etc, have to be put together to get the complete kinetic interactions.