Mention a sudden motion of the head and for a neuroscientist, the word concussion comes to mind.  The result is a scrambling of the electrical circuitry in the brain, producing some combination of headache, memory loss, confusion, blurred vision or unconsciousness.  In the world of professional fighting, where the objective is to knock out your opponent, this has been understood for a long time.  In professional football, it is a recent discovery.  Former NFL quarterback Terry Bradshaw, a Hall-of-Famer, believes that the sport will eventually lose its popularity as more and more concerned parents discourage their kids from participating.  When I was a kid growing up, smoking cigarettes was cool, sunscreen was for sissies and there were no seat belts.  The guys who got all the girls (and in retrospect, the STD’s) were the football players.  Things are different now.

http://www.nytimes.com/2012/06/21/sports/football/with-fears-about-safety-football-faces-uncertain-evolution.html?emc=eta1