I posted this challenge to the CTs four days ago and the only one willing to comment was one who believes the SBT. This is not at all surprising. Much of the CT skepticism about the WCR rests on their rejection of the SBT. One of the few things in the WCR that I disagreed with was their statement that the SBT was not essential to the finding that Oswald was the lone assassin. I agree with the CTs who have taken the position that without the SBT, there had to be a second shooter. However, the SBT has stood the test of time and it remains the only plausible explanation for JFK's non-fatal wounds and all of JBC's wounds.
Since most CTs reject the SBT and that rejection is so critical to their rejection of the single assassin, the question I posed to them in the OP is a real conundrum for them. JBC's sudden flip of his right arm at Z226, simultaneous with the beginning of JFK's rapid upward movement with both arms is an inconvenient truth for them. When I posed this question to Tony Marsh on John McAdams' forum his response was to simply deny JBC made any such movement of his right arm, which of course is ludicrous. It's there for all to see.
I would wager that most people, LN and CT alike, are even aware of JBC's arm flip. I'm sure no one on the WC noticed it. I first became aware of it when I saw it on David von Pein's outstanding website and I'm guessing that was roughly 10 years ago. When I saw that, any doubts I might have had about the SBT vanished. I don't know who first noticed that arm flip. For years, many LNs believed that JBC didn't react to being shot until the mid-Z230s which is when JBC himself believed he was struck. Since it was also believed that JFK was already reacting when he first reappeared at Z225, this forced them to believe JBC had a slightly delayed reaction. However, DVP's website dispelled both those myths. JFK was struck while he was behind the sign but his physical reaction didn't start until one frame after he reappeared. It also revealed that JBC had a two-stage reaction. First came the arm flip which was likely a completely involuntary reflexive response that JBC was completely unaware of. The second stage was the cognitive response JBC remembered making which was when he doubled over and twisted dramatically to his right, making almost a 180 degree rotation in his seat.
Maybe one of the SBT skeptics will eventually weigh in on this issue but I doubt it. Inconvenient truths are best left ignored when they challenge one's foundational beliefs.