The grassy knoll was forward and to the right, the opposite of the direction of JFK's head's motion.
If you agree that every action has an equal and opposite reaction, then his head moving backward and to the left may be inconsistent with a shot being fired from the front right.
My view is that the wound on the back of Kennedy's head was an entrance wound, and that the much larger wound on the front right side was an exit wound. I believe that the force of his brain and blood exploding out of front right side caused his head to move backward and to the left. The exit wound would have had a much more profound effect on the ultimate motion of Kennedy's head than the entrance wound would have, because the exit wound was so much larger.
An exit wound is always larger than the entry wound. That much is given. I admit I have a problem reconciling this to my theory. And, it's supposed to take only one contradiction to disprove a theory. On that basis my theory would be proven wrong. However, the seeming contradiction between the appearance of an exit wound from a bullet from behind, and the head appearing to move backward from a shot from the front would seem to indicate there is more at work here.
If JFK was shot from the front, why would there be an exit in the front of his head. However, I don't buy the force of his brains being blown out causing the head to move backwards (and to the left). And, if the head shot was from the back, why did his head not move at least somewhat to the front before moving back and left?
My theory of a gunman behind the corner of the grassy knoll only explains so much. I know there has to be more.