SB-
Thanks for posting, that is interesting. I have been aware for some time that JFK balls his fists more than clutches anything after being shot the first time, but you bring a clarifying perspective.
The more I looked at the frame Z-248 you provided, the more it looks like JFK actually turns and look to his own left towards Jackie. In this same frame Z-248 frame, Gov. JBC can be seen holding onto his Stetson hat, despite, by the SBT-LNT, having been short through the wrist with a large Western Cartridge 6.5 slug. A shot that left a large hole in the radius bone near his wrist, seen in x-rays.

I may post soon on what Secret Service agent Paul Landis belatedly related.
I have long had reasonable doubts about the SBT-LNT.
Some suspect the shot that apparently strikes JFK ~Z-220 never passed through his body, and cite the finger probe of Dr. Hume to the effect. Hume found only a shallow hole.
If Landis' account is accurate, one explanation is that the first shot to strike JFK was underpowered, and penetrated only marginally, and then fell out onto the back seat of the limo, where is was found by Landis. That is the real CE-399 slug, not the on fund by Tomlinson later.
Just IMHO.
Caveat emptor, and draw your own conclusions.