Knock Out

How do people in movies and TV shows know how to knock people out by hitting them on the back of the head? It doesn't seem like it should be so easy to knock people unconcious. I mean, I know how to "choke people out" which is sort of a misnomer since you're not cutting of the wind pipe and cutting of the flow of air as choking implies, but cutting off the arteries connecting to the brain thus restricting the flow of blood to the brain. People do it with such confidence in movies and such, it just doesn't seem right to me.