Okay what else would move Kennedy's head back in violent fashion at the EXACT same time his head exploded. Did you see the Zapruder film?
More times than I could count, and it's always been clear to me that the motion is not from the impact of a bullet. I guess it depends on one's expectations.
Bullets throwing people around is a Hollywood effect. In real life, super-sonic bullets move too fast and have such a small cross-section that they penetrate and pass through, imparting relativley little momentum to the body.
A rifle bullet also creates a "pressure cavity" inside the body. In Kennedy's case the bullet penetrated the rear skull and exited the front. The pressure cavity caused by its passage built up the internal pressure of the skull until it literally exploded. Like any explosion, there is an equal and opposite reaction from the direction of the expelled material. The ejected material went toward the front.
Also, Kennedy was not an inanimate object. He was a living human being. He was leaning forward at the time he was shot in the head. Why would you think he couldn't have simply jerked backwards?
It moved back violently at the exact time of impact.
Not really. The head moves slightly *forward* at the moment of impact, then the head and his upper body move backward after the head explodes and the bullet is long gone.
secondly the shot which penetrated his NECK from behind caused his body to move FOWARD
There is no such visible reaction at the time of the neck shot. That was just as he came out from behind the sign.