There are a number of ways, cutting blood to the brain, but I'm assuming you mean from a blow or blows to the head. When the brain suffers trauma from a punch in the head, chin, temple etc.The brain being the most vital organ protects itself by robbing the body of the oxygen rich blood, it says OH NO-& causes the rest of the body to pass out & down you go
Imagine this.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.