To understand the action of a helmet, it is first necessary to understand the mechanism of head injury. The common perception that a helmet's purpose is to save the rider's head from splitting open is misleading.
Skull fractures are usually not life threatening unless the fracture is depressed and impinges on the brain beneath and bone fractures usually heal over a relatively short period. Brain injuries are much more serious. They frequently result in death, permanent disability or personality change and unlike bone, neurological tissue has very limited ability to recover after an injury.
Therefore, the primary purpose of a helmet is to prevent traumatic brain injury while skull and face injuries are a significant secondary concern.
The brain is damaged during a crash when a rider’s head comes in contact with a stationary object which in most cases is the ground. The sudden deceleration of the head causes the rapid acceleration of the brain inside the skull. Under a hard impact the brain can strike the skull which causes unconsciousness, concussion and bursts blood vessels (bleeding of the brain) all of which take different amounts of time to recover from depending on the severity of the strike.