detonation is most frequently caused by hot spots in the combustion chamber. Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by the connecting rod the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the explosion) or down (the con rod) the piston will rattle from side to side. This is commenly refered to as "pinging"
Legacy is right. Otherwise, really neat things start to happen. On the lucky side, you only blow a head gasket. On the bad side, you blow the head, piston rings, etc...
