your ribcage will separate from your sternum on both sides absorbing some of the energy from a sudden stop if a racing harness is being used (a condition known as flail chest). Your heart has a protective sack around it that is filled with fluid that helps protect the heart from injury. The brain is the organ that usually will be the biggest problem, once the brain bounces off of the inside of the skull and the swelling or bleeding starts there is nowhere for the brain to expand since it is encapsulated (so then bloodflow gets cut off to parts of the brain and tissue dies, or even worse the brain swells to the point that it pushes itself down throuh the opening in the base of the skull for the brain stem) sorry for the rambling.....