The bullet that hit President Kennedy in the head was an expanding bullet that exploded upon impact which, in turn, exploded Kennedy’s head. This bullet could have come from anywhere and caused the head to jerk backward upon 'explosion.' It is a muscle and neurological reflex. It is not the direction from which the bullet came, it’s the effect of the bullet inside Kennedy’s head.
You can probably still find online a video of the execution of Italy’s fascist WWII leader Mussolini. They shoot him in the head from the rear, and his head goes forward.
Chicago Mafia hitman
James Files says he made the fatal shot, from the grassy knoll, but he also says a 2nd shooter was in the building to the rear. Files didn’t say this for money, as he turned down an offer from Oliver Stone. Files said he used a mercury laced bullet. He also said he left as a trademark a bullet casing with his bite mark on it. In 1987 a man and his son were on the grassy knoll with a metal detector and found a bullet casing with teeth marks on it. The bullet casing's markings (head stamp) indicate it was manufactured before the year 1972.
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James Files - Wikipedia
“Let me tell you what we did at Quantico. We reconstructed the whole thing, the angle, the range, the moving target, the time limit, the obstacles, everything. I don’t know how many times we tried it, but we couldn’t duplicate what the Warren Commission said Oswald did.”
–Marine Corp Sniper Carlos Hathcock– quoted in Craig Roberts, Kill Zone, p 89-90.