True, but that looped clip shows Kennedy after the first shot was fired. He'd already slumped forward, and you can clearly see Jackie reaching over to him in reaction to it.
One thing the clip clearly does show, is the violent backward motion of JFK's head at the moment of the second shot. A kinetic force applied from the rear is physically incapable of causing that reaction. If the head shot came from the rear, JFK should have been propelled forward, which we don't see.