In my own son's case, he nutted a tree trunk with a glancing blow but the bike somersaulted hard enough that we found one of his bidons lodged in the hedge about five feet up the bank. The "brow" of his helmet bore scuff marks and the side was broken; he had some scuff marks and bruising to his jaw and temple. If the blow, with helmet, was hard enough to KO him for a couple of minutes I shudder to think what would have happened without a helmet. I have also gone OTB hard enough to leave an imprint of the texture of the road in the helmet shell and for me to see stars. Again, I shudder to think what would have happened if that chunk of styrofoam hadn't been there to absorb and spread the impact.