This is presuming he can't hold onto anything. If he keeps his position during a stop, his head will whiplash after into the back door.
What I find funny is that we know what happened and we still have people arguing it didn't happen.
If you are bracing yourself against a deceleration you may sway back a little when the deceleration stops. I would be surprising if you braced yourself with sufficient force to throw yourself backward in the opposite direction with enough energy to fracture bones.
1. If you have a car that is breaking rapidly, there will come a point at which the tires grip completely, but the suspension continues to travel, loading up the springs as the weight lurches forward. The suspension then releases that energy, throwing the occupants backwards. Try it. Try slamming on your brakes hard and coming to a full stop suddenly. If your car does NOT rock backwards with a fair amount of speed, then you need to change your springs.
But the only way to make someone hit the back wall is acceleration.
2. which is very easily accomplished with very little torque if the person is already unbalanced from the braking and is shackled, unable to brace when thrown back.
3. Actually, it is not the only way. Another way is for them to take him out of the car, then shove him back in, but not all the way, and slam the door on his head.
4. And no, it's really not. You put the car into reverse, accelerate moderately, then slam on the brakes while traveling backwards. This applies all of the force and energy of braking to the body moving toward the rear of the van.
The defense has its work cut out for it in trying to come up with a story that is _more_ plausible than any of the above; all of which demonstrate deliberate action known to cause injury.