I guess it falls under the old adage that all models are wrong, but some are useful. Showing a cartoon of a brain rattling around in the soft space between the brain and the skull sort of gets the point across that some applied force is the genesis of the injury. Beyond that it's sort of worthless. When you think about the types of injuries that can result from a blow, it's starts getting complicated, because they could be compression, torsional, shear, etc. It all depends where on the head the force was applied and with what impulse, etc. For example, it's really easy to get a concussion from getting elbowed in the chin. You see it in sports all the time where a harmless looking bump on the chin keeps a guy sidelined for weeks. On the other hand, it's probably a lot harder to get concussed if you get hit in an area of your skull that is thick and also doesn't lead to a severe torque on your neck. Applied force is a prerequisite, but it's not even close to the whole story. In fact it's a small part when you look how many times people get hit in the head really hard and don't suffer a concussion or TBI. I've had two in my life and I've been playing ice hockey for more than 30 years. Force is a starting point, not an ending point.