The general consensus is that the Limo did slow down. All the films I've seen of the head shot show this.
I'm guessing Royal thinks the Limo stopped.
So what speed was the limo going at the head shot, perhaps 5 miles an hour? Walking speed is 3.1 miles an hour.
Is that enough speed to account for the head snap due to the momentum of the car going forward and JFKs limp head falling backwards?