ExploderWizard
Hero
I think people are over thinking this.
You attack the orc. At some point you decide you don't want him dead, you just want him incapacitated. You say so to the DM and/or the other players.
If you decide this before you reduce him to zero hp, there's no problem. Any combat description that occurs will take into account your intentions, and any attack you don't bother describing can be presumed to take into account your intentions.
If you decide this just as the DM announces that the orc has reached zero hp, you say, "Wait! I don't want to kill him, just knock him out!" and the DM says, "ok." And then you or the DM describes you knocking out the orc.
Its not like the order that things happen at the game table translates into an actual progression of time in-game. If you want to retcon the attack roll you just made to deal non lethal damage instead of lethal damage, the world won't crash down. Gamers retcon things all the time in D&D. "I'll run over and smash down the door." "You mean the massive, iron reinforced, 18 inch think stone door with the winch to open it?" "Oh. Uh, I don't do that then. I go and stab the orc standing by the winch."
The rules are allowed to take into account the human interaction aspect of RPG gameplay.
It works just fine as long as the rewards and consequences of victory through life vs. death remain the same.