With one good swing from your bludgeoning weapon of choice, you can hit like a locomotive and knock somebody's lights out. Make a called shot to an opponent's vulnerable spot, like the temple, jaw, or liver. If you hit, make a Damage Roll, opposed by the opponent's Guard Roll as per usual. If you win the opposed roll, the opponent takes no damage, but instead gains the unconscious condition.
In this case, Size matters. You can add +5 to your Damage Roll for every point of difference in Size if you're bigger than them. However, this also means that you subtract −5 from your Damage Roll for every point of difference in Size if you're smaller.