Kennedy's head moving 2.3 inches toward the front on impact. Notice the spray also going toward the front from the rearward shot.
You need to show more frames before and after. My recollection is his head distinctly moves backwards after the head shot. Could be a few frames after what you depicted. Not sure.
This clip shows only the instant of impact and the initial explosion of JFK’s head. JFK’s head was already moving down (toward the front) due to his reaction to the throat shot. Show another couple of frames and the bloody mist moves toward the left rear of the limo.......