Remember, the helmet is for the protection of the wearer and should not be used as a weapon or as the point of contact. Rules against butting, ramming and spearing with the helmet are for the protection of the helmeted player as well as the opponent. A player who does not comply with these rules, in any sport, is a candidate for a catastrophic or fatal injury. Helmets do not protect against concussion. For example, a player falling to a surface and striking the back of his head may get a concussion.
Concussion is the rapid deceleration of the brain as the head stops suddenly upon impact.
In this scenario, the helmet reduces direct impact to the head, protecting against internal bleeding. It does little, however, to buffer the rapid deceleration of the brain. The best defense against concussion is effective rule making and rule enforcement. For example, concussions have increased in ice hockey and football. In response, the sport rules committees have enacted tougher penalties for dangerous play that could contribute to concussion.