Usually used in very physical oriented sports. (ie: american football
Said when one athlete is hit very hard, and often falls down on his/her back, knees, is spun around, drops to the ground very suddenly, or has an injury from the hit, losing conscience, almost instantaneously after being hit. Could lead to concussions.
Very entertaining, but the viewer should feel guilty for watching.
Could also be applied to real-life collisions where one person falls down hard, and stays down for a period of time. Tripping and falling, etc.