The Dft is currently running an radio advert to promote wearing a seatbelt (http://think.dft.gov.uk/think_media/241045/241134/heartbrakev1.mp3).  The advert says that if I don't wear a seatbelt and crash my car I will fly forwards into the dashboard and my body will stop, but my internal organs will continue moving and be crushed against my ribcage.  The radio advert implies this will not happen if I wear a seatbelt.  Why not?  The seatbelt also stops me moving but not my internal organs, so why do my organs not crush the same way as if I hit the dash?