When a two-wheeler meets with an accident, the occupants of the vehicle are thrown down. If that person's head hits a solid object or something else on the road, the forward motion of head stops. But the brain, being its dominant mass, continues its motion and falls back against the inside of the skull. after which it collides with the opposite side of the skull.

→ A serious head injury in this type of collision can result in immediate death, but wearing a standard helmet can prevent serious head injury.
→ The main function of a helmet is to protect the head and brain from serious injuries by reducing the impact of collisions on the head.