The short version: you want to land a punch that hits one side of the jaw with the force going through to the other side. If you can land it out near the chin, so much the better. It's more a precision shot than a power shot.
You can apparently KO someone with a blow to the side of the neck, too. There are other pressure points that can do similar things. That old "Judo chop to the base of the neck" you see in old 60s spy movies actually works. It won't give you a total KO, but it can buckle your legs. Feels strange.