That is what the HSCA panel observed, although they had JFK’s “severe external stimulus” earlier:
Connally's movements as he emerges from behind the sign at Zapruder frames 222—224 also indicate that he is reacting to a severe external stimulus. He appears to be frowning and there is a distinct stiffening of his shoulders and upper trunk. Then there is a radical change in his facial expression and rapid changes begin to occur in the orientation of his head.
Orr has JBC being shot at Z236 and reacting at Z237, which does seem clearly incorrect even though he cites support from Six Seconds in Dallas and the three Parkland doctors who treated JBC (in terms of how JBC's body would have been oriented). It would seem to me that the critical question is when JFK was hit.