I blew up and slowed down some higher resolution video from the History Channel. You can clearly see him drooped forward when some sort of incendiary device came in from the upper right and exploded in front of Jackie’s face. Then it attaches to the front right quadrant of JFK’s skull where it burns brightly, and knocks his head sharply backwards. Jackie then pulls his burning head forward out of her way and immediately crawls out of the back seat.
The original History Channel video is below. Take a close look at the last ten seconds after reviewing the imagery above.