So, he is about to fire there. That is just a tiny fraction of a second before the gun goes off.
Now, nobody can deny that Oswald is reacting to being shot. But, Ruby's hand is in the same position as before. He's not going through recoil. And you can't tell me he already went through it. Here is a video of a guy shooting the Colt Cobra, where you can see the recoil, and he makes the comments, "It definitely kicks a lot harder."
But wait! That guy was shooting with two hands and from a very stable, planted position, where he was really braced for it. You make like a tree. Being so planted and having his arms straight like that really helps to control it. But, "Ruby" was shooting with just one hand. He had only one arm involved. And he wasn't stable and planted either, but on the move. Imagine the recoil he would have gotten.
You can watch "Ruby" in action. See if you see any recoil. Watch it in Google Chrome so that you can slow the speed down to .25.
I'm satisfied that there is no sign of recoil.