hmmm while that is certainly possible... injuring yourself to that extent seems a bit much.
Not at all. Your hands are cuffed behind you. The vehicle is stopped at a red light. You stand up, facing the rear of the vehicle. You are bent over because the vehicle is not tall enough for you to stand upright. The light changes to green and the driver starts through the intersection. You lose your balance and fall head first into the rear door of the vehicle.
Sounds to me very much like a typical paralyzing neck injury that happens to football players who don’t tackle properly.
There is all sorts of evidence that Freddie Gray was moving about inside the vehicle in just this sort of manner. In any case, there is enough “reasonable doubt” as to the officers’ intentions and actions to acquit all of them.