Everyone's head is built differently. Some people are bleeders, where a little jab to the nose has them gushing blood. Other men can take an overhand punch right on the jaw and barely react. They call this "chin" in boxing/MMA- as in, "This fighter is known for having an iron chin, he eats shots that put other men down." Your "chin" actually degrades every time you take concussive damage. Hypothetically, let's say I've never been KO'ed, and you and I are fighting. You're hitting me cleanly with volume punches, 40%ish power until you set up and throw a punch using about 70% of your power, and catch me clean- *lights out for me!* I go down, limp, and start coming around as I hit the ground. 3 months later, you and I are sparring again. This time, you are throwing your 40%ish volume punches, some are catching me in the head- and I keep getting flashed. Before that KO, these punches shouldn't phase me this much- but I've lost some "chin", so eventually you catch me with a 50% punch and it floors me again. You can see this in pro fighters. Usually after they take a bad beating+KO, they tend to get KO'ed a lot more frequently.