Why do you get knocked out?

Answer 1

Your brain is inside your skull like a box containing some breakable stuff.

When you get a good hit somewhere on your head, particiularly your jaw, your brain is shaking inside your skull, just like a ball in a pinball game.

Remember our box with breakables? Imagine what would happen to the stuff inside if you shaked it real hard. That's what's happening to your brain and that's causing the knock-out.