Very convincing to a rational observer, unless, of course, Greer just happened to slam on the brakes at that very moment, or ... or ... or ...
IF THE ZAPRUDER FILM WAS ALTERED!
-- MWT
Even more convincing to a physicist, I shouldn't wonder.
Take another look at my post and notice I've added a gun nut firing a Carcano and which just happens to confirm the equal and opposite reaction of Kennedy's head movement. Reminds one of Sir Isaac Newton's 3rd Law (hey, another 3!) of Motion, eh wot?