I've always seen it as them putting their hands up, probably dropping a weapon if they have it.
It's certainly not them falling unconscious, which is what helpless leans towards. I take them off the tracker, but not off the board because they're still an element of the combat.
The intimidator basically says, "See how awesome we are. Knock it off or we'll kill you." When the PCs start making it obvious they're going to kill the guy anyway, the enemy is probably going to rejoin the fray.
This.
I would ordinarily "remove" them from the initiative order too, but if the PCs then attacked the surrendering creature again, it would rejoin the fight. After all, what's the point of surrendering if they are gonna kill you anyway? You surrender in the hopes that they'll let you live.
As an aside, I would probably talk to the other players if I were you and give them a head's up as to what you are planning to make sure that they understand and are willing to cooperate. This likely includes not killing the captives after you interrogate them as well. After all, your party will eventually develop a reputation for killing prisoners if there's any kind of connection between adventures. Again, a combatant is going to surrender in the hope of being allowed to live. If its clear he'll be killed anyway, then he might as well take his chances.
which means that a roll of 17 will succeed in this instance without further bonuses. Bizarrely this means that as you reach higher levels it becomes harder to intimidate with Will going up at +1 per level while the attack bonus only goes up at + 1/2 level.
), means that the chances of success are fairly reasonable. For a half-orc of a class with intimidate on the skill list it would be even better (I'm assuming that there is a suitable item for such a character which would give the +2 bonus without actually checking for one