Why is the minute movement forward evidence of a bullet from behind, while the violent thrust backwards is NOT evidence of a bullet from in front?
Firstly, you are making the same mistake that Jim Garrison made....
"Back and to the left" is NOT, repeat NOT always indicative of a shot coming from
"in front and to the right"
Secondly, you can't take evidence in isolation like that. You are trying to deal with evidence one bit at a time in a complex and dynamic situation and it does not work. In such situations, one piece of evidence often relies on other pieces of evidence.
The slight forward movement is evidence of the initial impact of the high velocity bullet from behind; most likely caused by muscle contraction or nerve response. It could also be the initial energy of the impact... high velocity bullets carry a LOT of energy. It could even be a little of each.
The large backwards and to the left movement is evidence of the explosion of the pressure cavity in the front right created by the tremendous loss of energy in the millisecond or two after the bullet impacts. That energy cannot just disappear, it has to go somewhere, and where it goes is into the creation of a pressure cavity in the brain case. Its as if a small piece of explosive had been set off inside the front right part of his skull.
Now, if your claim is that he was struck in a head by a shot from the grassy knoll, you have to provide some physical, ballistic and forensic evidence, and this evidence will be conditional on what you claim happened.
1. If the shot was fired from a low velocity weapon such as from a small calibre rifle, say a .22 cal, or a pistol, then the bullet would have remained in JFK's head. So where is it? Why did it not show up in the X-Rays?
2. If the shot was fired from a high velocity weapon such as a .270 or a .30 cal, then the bullet would have created a pressure cavity in the back left of JFK's skull, and there would be a large exit wound caused by the pressure cavity. So where is this large wound on the left side of JFK's head?
- There is no evidence of it on the Zapruder film, or any of the other films.
- There is no evidence of it in any of the autopsy photos
- There is no evidence of it in any of the X-Rays
- There are no reports if it or statements about it from any of the "gore" witnesses
3. Where are the bullet fragments? If there was a shot from the grassy knoll (which is a slightly elevated firing position), then the area beyond the firing zone where any bullet fragments are likely to end up would include the back left of the presidential limo and the grassed area on the north side of Elm directly opposite the Bryan Pergola. Any bullet fragments that passed through JFK's head would have very low velocity (lost all their energy) so they would be relatively easy to find in the firing zone... probably exposed on the ground or at worst, a couple of inches under the ground. That whole area was searched... nothing was found. The only bullet fragments from the kill shot were found in the front area of the limo.
In short,
- there is no physical evidence for a shooter on the grassy knoll
- there is no film/video/photographic evidence for a shooter on the grassy knoll
- there is no ballistics evidence for a shooter on the grassy knoll
- there is no forensic/medical evidence for a shooter on the grassy knoll
ergo: there was no shooter on the grassy knoll.