It's the other way around. You see him pull backwards and ASSUME it must be from a shot. But it doesn't have to be, and isn't consistent with that. A shot to the head would move the head. Instead his whole upper body pulls back.
There is however a small sudden *forward* movement to the head at the instant of the head shot.
Wow. What film are you watching? The only movement the head makes is a snap backwards and to the left, then he slumps towards Jackie.