[Pistolshot]

I don't believe the sudden jerk to the rear is any evidence of a shot from the front. The brace he wore during trips was known to be restrictive and allowed little free movement of his upper torso.

The head shot destroyed a massive amount of brain function instantly, all motor skills cease and the tension of the brace snaps him to the rear. There is ample evidence of the brace and the fact it was being worn in Dallas.

As you watch the moment in Zapruders where the fatal headshot takes place, look at his hands and the reaction after being struck. the wound to the neck has already produced the 'Thornburn reaction' and Jackie is trying to push his arm down as he is struck by the fatal shot. All muscle function ceases, his arms drop and he slumps to the left.

He wore a back brace. For example, on the day he was shot, he wore a brace that consisted of a canvas brace with metal stays, together with an Ace bandage with extra padding. Dallek, Robert. The medical ordeals of JFK. Atlantic Monthly. 2002 (Dec);290(5):49-61. [a] p. 61 [b] p. 60
Back brace. Markings on the brace that President Kennedy wore indicate that it came from the Washington, D.C. firm of Nelson Kloman Surgical Supply Company. John F Kennedy Presidential Library.

[Tublecane]

“the wound to the neck has already produced the ‘Thornburn reaction’”

As I understand the Zapruder film, Kennedy’s reaction to the shot is not evidence of the “Thornburn position.” The bullet passed near the cervical part of his spine, but caused mostly tissue damage. Moreover, the classic Thornburn pose, features upstretched arms, whereas Kennedy’s arms bent inward at the elbow. Lastly, you can see Kennedy’s arms relaxing from the position even before Jackie leans over to help him, whereas textbook patients have their arms locked for days.

[Shooter 2.5]

Frames 312 and 313. The bullet impact from the rear causing the two inches of head movement toward the front.