They are looking for head trauma that is potentially complicating or fatal when they CT scan you after you bang your head. Internal bleeding, fluid on your brain, etc. Except those are conditions that generally do not coincide with the patient 'feeling fine', so if you have sustained such an invisible injury, don't worry, you'll start feeling crappy soon enough. The awareness that any such symptoms should get you to the hospital to get checked out quickly should be enough, I think the odds of that happening if you feel fine 5-12 hours after you bang your head are <1%. If you're injured your brain, you'd start feeling disoriented, nauseous, headachy, etc. which would clue you in to take action.

On the other hand, BF injures my brain all the time and I still feel fine.