The best way to incapacitate another combatant without causing them undue harm is to attempt a Grab, immobilize them, and then begin a choke (see above). However, that's a maneuver that takes some skill in hand-to-hand combat.
On the other hand, most creatures have sensitive organs. One good swing of a bludgeoning weapon to the liver or to the side of the head will drop most folks. This stunning weakness of anatomy can be exploited in combat to subdue your foes while keeping them alive… probably.
To attempt a knockout, make a Called Shot to the sensitive location in question (a −5 penalty) using any bludgeoning weapon, including hand-to-hand. If you hit and deal damage, the opponent must make a Stamina check at a DL of 10 + any damage dealt. If they fail, they move one step down the Knockout Track. On a Critical Failure, they move all the way down the Knockout Track and immediately gain the unconscious condition.
A character with the Haymaker Trump causes any foe who fails the Stamina check to drop immediately, instead of just those who critically fail.
In this case, Size matters. Opponents take −5 to the Stamina check for every point of difference in Size if you're bigger than them. However, this also means that they get +5 to the Stamina check for every point of difference in Size if you're smaller. If an opponent has a large enough bonus that they would pass the check even on a Critical Failure, you can't knock them out in one hit.