I have a few thoughts:

First, you can see people in the crowd duck in the video. Their reaction time isn’t very good, but a big events people can be slow to react due to confusion and a desire to not stand out from the crowd. It’s very clear they got down in the front on video:

Second, while I do believe your theory has some possibly of being true, it would be a weird tactic for someone who is the consensus front runner and whose opponent is a sitting president with dementia. Bidens own team is turning on him, so to pull a risky stunt like that would be a very strange move in my opinion.

Third, Trump is likely not thinking when he fist-pumps. If this were a real situation and not staged, there would likely be a lot of adrenaline which would be making his decisions turn into fight or flight reactions. Since there is no one to really fight, aggressively standing up and celebrating not being dead makes a lot of sense.

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