I really don't see the importance of a clenched fist. The obvious is his reaction like someone choking or about to gasp for breath. The motion exhibited show the President in a motion to move his hands towards his neck and the continuance of the frames shows that. There is an obvious reactive movement to something foreign that just occurred. He normally would be continuing to wave. To me, he felt something hit him and instinctively moved hands to neck area as someone who is choking. Frames later you can see his puffed cheeks which indicates breathing problems are occurring. I would agree the shot came while he was hidden behind the sign and he changed his stance from crowd waving.
I would be more apt to analyze how high his right elbow goes up in reaction. For example, is this a defensive posture move as a bullet comes from grassy knoll direction to protect himself from a further assault? His first reaction is not to duck and lower his head behind the seat. Jacqueline quickly reaches over with both hands and puts her hand on his left arm to stabilize him. The secondary movement of the crowd running to the grassy knoll should give secondary confirmation of where bullets where coming from. Who benefits from a single lone nut gunman narrative that is contrary to the crowd's response?
Reactions at or after Z313 by others in car are much more extreme but coincide more with a frontal/side assault than a rear assault. Connally and front SS agent duck below seat level. JFK's head move back with impact not forward. Connally's head reappears behind driver's post in an obvious defensive move to move left and out of harm's way in the shuffle. He is still very conscious of his movements and appears to be sitting or kneeling? there after the assassination was completed - his head wasn't in Nellie's lap yet!
In fact, the movement exhibited at Z313 by the President is an exact same movement to the first shot. His right elbow raises once more, just weaker. This time it is a headshot but has exact same reaction exhibited by him. Notably, the bullet also strikes a hard object (his head) and pushes it back.
The other thing to note is that if this was a mafia hit job and not an inside coup d'etat, you would expect more transparency and a real investigation to ensue. There is nothing really to hide if it was the mafia and so why were there all the investigative problems/blunders and total lack of congruency - abetted confusion? These people are not a bunch of bumbling idiots like they would have you to believe! A totally controlled event including autopsy control, destroying or tampering with evidence , pristine bullets on stretchers found for example and creating statements and narrative as required. This was total control, well planned and executed. Car and body removed from Dallas and information released as required to paint a story. Time is a great healer and so films hidden and released years later. 50 year seal orders placed to protect the guilty. Most evidence really was shredded anyway and nothing left to hide as it was sanitized early on. Why the need to protect if a LNer or even if mafia?
This is a classic example of state controlled media and events - much of what we continue to see today. Patriot's Act and Homeland Security Act which enacted to allow government to spy on its own people at will. Was it politicized and used against Trump and his campaign and presidency? 2nd amendment rights are on the way out if they have their way and COVID used as a weapon to create unrest and violence and aid that end. The scaremonger mentality is being pushed through once more with the help of media. Do the people control the government or does the government control the people? A lot of scientific expert opinions .....
I digress! Clearly, currency is backed only by paper and not gold. This started with FDR and Executive Order 6102 which banned people from owning gold! Do the people of a nation benefit from such moves and erosion of their rights? JFK saw this happening and called it the Military-Industrial Complex - Crony Capitalism for short. Trump like JFK has no business interfering with their games and exposing the swamp and how they get rich and ride the wave. Why do those in politics end up getting rich? Did past Presidents Obama, Clinton, the Bushes and others make their money from Presidential wages or was it by other means? Obviously you could launch an argument that they were just shrewd investors and used their wages "earned" to get rich - that's a laugh! Did Biden give his brother choice military contracts in Iraq?
Democracy is nothing more than a facade. Families run countries. Whether Democracy, Communism or Socialism, the scum always floats to the top, rich get richer and poor get poorer and politics has an "end". Again sorry for the digression.