I just wanted to break this down for people who've never been in a fight before (knowledge is power!) It's hard to tell with such low resolution but the reason hes KOed is because he was punched in the jaw at an angle (cuz the guy's taller, it makes it much easier). The brain kind of blue screens when that happens with the correct force.

Remember kids: guard the jaw. Bonus tip: the forehead is incredibly strong.

P.S. running away should always be priority number one. If thats not possible, then you fight