GURPS has two broad circumstances where you roll against HT in combat to stay active and effect - avoiding unconsciousness (see General Injury: Lost Hit Points, p. B419) and Stunning and Knockdown, p. B420.
This GURPS 101 looks at these and gives some examples to make it clear how they affect you. They might change your mind about how much HT you need, as well!
When your HP reach 0 or below, you must make a HT roll at the start of your next turn or fall unconscious. At -1xHP, you roll at -1, at -2xHP it's -2, down to -4 at -4xHP. At -5xHP, you're dead, so you don't get to roll.
Note that this roll occurs on the turn of the injured person, not when they are injured. In other words, if A attacks C and puts him to 0 HP or below, and then B attacks C and puts him to -1xHP, on C's next turn C rolls once against HT-1 to stay conscious.
Going unconscious in a fight is mostly a clear-cut bad thing; you're out of the fight and at the mercy of the rest of the combatants. On the other hand, if you're very injured it can be better to pass out and give your foes a chance to move on and not keep beating on you. Foes with Bloodlust and animals seeking to devour you, though, taking away that small grace.
"You stunned him, just as he was wakin' up!"
- The Parrot Sketch, Monty Python's Flying Circus
Stunning and Knockdown
Unlike rolls to stay conscious, rolls against Stunning and Knockdown are rolled immediately, along with the injury inflicted.
This can get a little tricky; remember that stunning and knockdown is one roll. If you're stunned, you drop prone (and drop whatever you're holding), and you are stunned.
When to Roll:
- any major wound (injury in a single blow equal to more than HP/2); or
- any crippling injury (see (Effects of Crippling Injury, p. 421); or
- any injury to the head (skull, face, or eye) that causes a shock penalty; or
- any injury to the vitals that causes a shock penalty.
"Causes a shock penalty" effectively means (HP/10, round down) injury. It doesn't matter if you actually suffer shock penalties or not (thanks to High Pain Threshold), you must still roll if the damage is sufficient to have inflicted a penalty. High Pain Threshold does provide a +3 to the roll, and you don't take any shock penalty, but you can still be stunned or knocked out. And since it's possible that you could be knocked unconscious from this roll, Hard to Subdue provides a +1 per level to resist Stunning and Knockdown.*
Major wounds to the face or vitals are a -5, skull and eye major wounds are at -10. Also, it's important to remember at a failure by 5+ causes unconsciousness, not just stunning!
Those under the effects of the Berserk disadvantage are immune to stun; they still roll, however, with a +4, and only a failure by 5+ affects them at all, by knocking them out!* Those with Injury Tolerance that effectively removes some hit locations remove the special penalties for those locations - No Brain means never rolling at a -10 for a major wound to the skull, for example.
When you're stunned, you must Do Nothing and then roll at the end of your turn to recover (per Effects of Stun, p. B420). If you succeed, you are no longer stunned. If you fail, your turn ends and you must do the same on the turn following. (Special 3e alert - the roll used to be at the beginning of the turn, so if you recovered you could also act. This is not true in 4e!)
Example: Hjalmarr has HT 13, HP 20, and High Pain Threshold. His foe stabs him in the face and the vitals with a pair of attacks on a single turn. The stab to the face does 2 HP, just enough to inflict shock. Hjalmarr must immediately roll against HT 13 + 3 (High Pain Threshold) = 16 to stay conscious. He rolls a 12 and makes it. The stab to the vitals inflicts 12 injury, enough for a major wound. Hjalmarr must roll against HT 13 + 3 (High Pain Threshold) -5 (major wound to the vitals) = 11 to avoid stunning. He rolls a 15 - failure by 4. He's stunned and knocked down. Had he rolled a 16+, he'd have been knocked out! Instead he just drops prone, and his axe and shield** tumble from his hands. On his next turn, he must Do Nothing and will roll against his HT (with no special bonuses or penalties) at the end of the turn to recover from stun. He'll be unable to retaliate for at least one second!
So a major wound to the skull or eye is at -10. Even your HT 15 monster with High Pain Threshold rolls is rolling against an 8 to avoid stunning, and on a 13 or higher (failure by 5) you are knocked out. An average person (HT 10) would be rolling against a 0 - you're stunned on any roll and pass out on a 5+. With an injury multiplier of x4, this means even a low amount of penetrating damage to the skull or eye can incapacitate a foe!
The penalty for staying conscious at -1xHP (below) does not apply to Knockdown and Stunning; while that might make an interesting house rule it's not the RAW.*
* These were confirmed with Reverend Pee Kitty, Assistant GURPS Line Editor
** Clearly a buckler, since you only drop what you are holding.