This article from the CBC illustrates just how easily a brain injury can occur. In a reality is stranger than fiction story, a heavy metal music fan ended up with serious brain injury after engaging too enthusiastically in “headbanging”. According to the article, this is one of three confirmed cases where a brain injury occurred after headbanging. The injured party in this case may have had a pre-existing benign cyst, which left him more vulnerable to a head injury.
Despite not suffering from an external head injury or blow to the head of any kind, head injuries, such as concussions, can still occur and be life threatening. This illustrates how the quick stop and go motions from motor vehicle accidents can result in severe injuries, despite their being a lack of external physical evidence. The injury in these cases is caused by the brain impacted the inside of the skull.
The Brain Injury Institute wrote an excellent article on how several common forms of motion in car accidents can lead to head injuries, including: airbag impacts, windshield impacts, and simple whiplash.