When one is struck in a rear-end collision, the head normally thrusts backwards and strikes the headrest, then it whips forward and back again. That whipping motion can cause rotational forces in the brain that result in a traumatic brain injury.  The victim may have a classic whiplash injury that is often resolved after a course of physical therapy and/or chiropractic treatment.  But if the forces are great enough, the victim might have a severe concussion/traumatic brain injury.