Creative Speculation

The other people in the car do not snap forward the way JFK's head does.

> And, your thesis does not explain how the forward momentum of a bullet from behind throws his head back as we see but a moment later.

Again, the point is, the "back and to the left" cannot be explained by momentum from the bullet because (A) it doesn't start until two frames after the hit, whereas momentum transfer must be simultaneous with the bullet losing velocity, and (B) that motion shows acceleration for several frames, which implies that some force is still acting on the head and shoulders, which can't possibly be the bullet.

And again, it isn't necessary to explain the "back and to the left" to say that it wasn't caused by the bullet hit, but there are several explanations available. I believe the most plausible explanation is that it's simply a jerk -- a neuromuscular reflex reaction to being hit by a bullet. While the infamous Alverez "jet effect" is based on real physics and must have contributed some backward force, it doesn't appear that the "jet" continued long enough to explain that acceleration for several frames. The compression of the spine itself would cause some backward force, too, sufficient to stop and reverse forward motion, but that also doesn't explain the acceleration for several frames after that.