One thing I've learned (I'd probably forgotten) in the last two days (thanks to the internets) is that by age 40 all the sutures of the skull are fully knitted such that the skull is essientially a more uniform structure. As a consequence, force applied on one area may transfer to another area of the skull as opposed to being localized. Post head injury, the impact consequences of the blow may not be initially detectable.
I've also learned that localized injury (back of the head for example) can affect very specific nerves.
Here's an interesting onservation. accidentally ended up at a climbing helmet site. What surprised me is that the site mentioned a 2 meter drop test for helmets. I understand the 2 meter height in bikes based on the "average" cyclist head being 2 meters above the ground but no explanation was given for 2 meters climbing helmet drop test. Climbers worry most about top of helmet rock fall but they also worry about side strikes if they fall. My friends who climbed (technical rock) always worried about an aggressive lead climber getting to far head of his protection. 3 meters above your protection can becomes a brutal 6 meter arrest (@ up to 39 km/hr)......if your protection holds.
I rode motorcycles since age 12 so using a helmet on a 2 wheeled bike was second nature and part of the gear. Helmets/face shields and hard hats have likely protected my head from some nasty scalp/head injuries. Like I noted earlier...I've learned exactly squat from posts on this thread with one exception...reading the links eventually gets you to real data after a lot of messing around.