In HEAD INJURY : WOULD A HELMET HAVE HELPED?, bicycle accidents expert witnesses at Walters Forensic Engineering write:
In general, brain injuries caused by impacts are produced by one of two mechanisms. Sudden deceleration is one of these, and in such a case the brain is injured due to its motion inside the head, including collisions with the inside of the skull. The notable feature of this kind of injury is that the brain may suffer serious trauma even where the skull remains undamaged. The other mechanism of brain injury occurs when the skull is cracked or penetrated, in which case brain injury can occur regardless of the level of head acceleration during impact.