I can see your point. To me, I don't necessarily see a fist clenching. Camera angle may dictate hand disappearing from view. The left hand is reacting at the exact same time - a reflex movement. One can see before the sign that the President is giving a last crowd wave as the right hand is still up and he has no distress. The crowd is thinning and it appears his hand moves down from his upstretched position somewhere behind the sign. Certainly I think your analysis is correct and it is a reflex action rather than a premeditated one and is much quicker than that of his wife - reflex versus reaction.
So compare his reaction time to that of Jacqueline who appears to be looking at him the whole time. She has a reaction time of about 8 frames. That is a reactive motion to an external stimulus. Use that reaction time to make further analysis. At frame Z338 she appears to have had quite enough stimulus and gets ready to leave the crime scene. Given the logic that 8 frames or half a second is about an adequate normal human response time and count from there backwards. She is looking right at the President the whole time and appears to be studying him after the first shot so she is on heightened awareness. A massive blow with fragments flying all over right in front of her face at Z313 should lead to a major reaction at Z321 (1/2 a second). My feeling is that reaction time of 1/2 a second is a realistic estimate to external stimulus by her or any human. Logic should say that a reaction by her at Z338 would coincide with a messy kill/headshot coming in at Z330 (1/2 a second earlier) with glass shards catching the sun light over a ducked Connally and SS agent and JFK's head a red ball. In my opinion, 2 seconds is an unrealistic reaction time to a supposed shot that totally removed the President's head and she misses it for 1 1/2 seconds even though she is looking right at him. Just compare her initial reaction at the first shot that strikes him to the despair reaction at Z338. No doubt she thought the next bullet was for her!
The facts are we know the car and body was whisked away in an un-examined state from the crime scene and that a windshield was changed and a controlled autopsy was done out of state - those facts are undisputed.
Was thinking about the third shot. I was speculating that this shot hit the corner of a concrete slab surrounding a manhole cover on the south side of Elm Street, as reported by Dallas officer J W Foster:
Mr. BALL - What did you do after that?
Mr. FOSTER - I moved to -down the roadway there, down to see if I could find where any of he shots hit.
Mr. BALL - Find anything?
Mr. FOSTER - Yes, sir. Found where one shot had hit the turf there at the location.
Mr. BALL - Hit the turf?
Mr. FOSTER - Yes, sir.
Mr. BALL - Did you see any marks on the street in anyplace?
Mr. FOSTER - No, a manhole cover. It was hit. they caught the manhole cover right on the corner and -
Mr. BALL - You saw a mark on the manhole cover did you?
Mr. FOSTER - Yes sir.
Mr. BALL - I show you a picture here of a concrete slab. or manhole cover. Do you recognize that picture?
Mr. FOSTER - Yes, sir.
Mr. BALL - Does the picture show - tell me what it shows there.
Mr. FOSTER - This looks like the corner here where it penetrated the turf right here [indicating].
Mr. BALL - See any mark on the manhole cover?
Mr. FOSTER - No, sir; I don't. not on the - well, it is on the turf, on the concrete, right in the corner.
After hitting the concrete the bullet fragments, part of which goes on to hit the curb near Tague causing the injury to his face. The problem with this scenario is that the assumed sniper has hit JFK with the first shot in the back (I assume this was meant to be a head-shot). The next shot is the head-shot which indicates that the sniper is getting his eye in. But then the third shot seems to be pulled high. Why a third shot? The sniper must have seen JFK's head explode. And why did it miss if he has two out of two already. Then I read what you posted about Jackie Kennedy and I had an idea.
This is speculation. It's based on asking what has changed between shot two and shot three to make the sniper miss. The sniper is just about to take the third shot and suddenly
Clint Hill is in his sights! Shooting a member of the SS is not in the plan. The shot is pulled high, fragments on the concrete of the manhole cover, a piece of which goes on to cause the injury to Tague.
Obviously total speculation but something I find interesting and worth pursuing.
This pic apparently shows an unidentified man (SS agent??) reaching for possible bullet fragment near manhole.
Possible bullet strike on concrete of manhole cover in the position described by Foster in his testimony.