Kennedy's head didn't move in the same direction as the bullet. Conspiracy buffs keep saying: "Backward and to the left." Oswald's third shot was fired toward Kennedy from his (Kennedy's) right to his left (i.e. approaching from over Kennedy's right shoulder). If Kennedy's head had moved in the direction of the sixth-floor window, it would have been backward and to the right.
For every action there is an equal and oppopsite reaction. Kennedy moved backward and to the left while his brain and blood exploded out of his head forward and to the right. As far as I'm concerned, this explains why he moved backward and to the left.
Yes, specifically it did go backward and to the left. That's indisputable. And, yes, the 6th floor SDB window was back and to the right. However, Kennedy's head went backward and to the left, which leads to...
" For every action there is an equal and oppopsite reaction."
The grassy knoll was forward and to the right, the opposite of the direction of JFK's head's motion. After having personally walked all around behind the fence that's behind the grassy knoll, I feel the final shot came from behind the fence at the corner up by the railroad tracks.
And, my first visit behind the grassy knoll was in the early 70's when it was still a gravel and dirt rail yard and before the new parking lot and other changes to the general area associated with the West End.