Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

I found an answer on Quora to the question of whether the momentum of a bullet can be transferred to the body as a whole, causing it to move, and the short answer is NO. 

https://www.quora.com/Does-shooting-someone-really-cause-them-to-fly-backwards

And I note that some of those responders have extensive military or law enforcement backgrounds. while others sound like they have taken a Physics course or two, or even taught one. So, what was shown in the Ruby and Oswald film, where Oswald was flown backwards was entirely fiction. 

Let's hone in on this answer, although it says nothing about the author's bio:  

Tim Gordon

Written Dec 1

Does shooting someone really cause them to fly backwards?

In some circumstances, sort of.

The bullet or shot column isn’t going to be what moves them. They’ll be moving under their own power if any sudden movement occurs. Some people can be caught off balance at the time of projectile impact, and this will exaggerate the movement that they make, if any.

Odds are good that any movement that you see was a move they were about to make when they saw a weapon pointed in their direction: retreat, duck, dive, turn, etc. The bullet hits them at just about the time the nerve impulses get where they were going in order to make those movements possible. In this instance, the movement was coincidental rather than a result of being struck.

Involuntary muscle contractions can sometimes do the same thing. This depends to a large degree on just where the bullet hit. If it impacts a large nerve bundle, you can expect quite a reaction.

Alright, Oswald got shot in the side, but there is no reason to think the bullet should have driven his body to the side. We have to consider the direction he was moving when he was shot, and he was moving forward. And Oswald's initial reaction to being shot was to crumple forward. But, what explains his subsequent veering backward and going up on his toes? Obviously, the bullet did not cause him to do that. HE did it. And, there is no reason to think it was due to involuntary muscle contractions. That's ridiculous in this case. So, the question is WHY did he do it, and secondly, HOW did he do it? How, as in how does anybody whose aorta and vena cava been blasted through change direction and do something else?