Dick,
The usual explanation I've heard is that his head snapped forward while it was behind the sign and then backward immediately after that -- at the same time that it reappeared in view.
One reason why the head would do the forward then backward thing is that the head is mounted on a shaft (neck) and hinge (where spine dips between shoulders.) There is a mechanical tendency of things so mounted to swivel violently to their forward extreme of motion and then snap back.
Fred
|