Whenever I read about a car accident involving decapitation, I imagine the person slipping under another vehicle or some car part cutting off the head.  Well apparently,  that is in most cases an incorrect assumption. 

My mechanic, who races cars, was explaining the phenomenon to me while giving me a lift to work.  I asked him about the thrills and spills of car racing and it came out that he bought himself a special harness for his head that cost close to a thousand dollars.  I inquired about the need for the apparatus and he offered the following explanation.

What happens is when the car hits something your head is propelled forward and your body is held back by the seat belt. Due to the weight of the head, (about 25 lbs) it continues with momentum in the trajectory of the collision. This causes the head to rip apart from the body causing severe injury and eventually death. 
This would explain why my father in law, a retired police officer, never wears a seat belt.