How does the Barbarian rage power Guarded Life function, exactly?

Question

Barbarian's Guarded life rage power states:

While raging, if the barbarian is reduced below 0 hit points, 1 hit point of lethal damage per barbarian level is converted to nonlethal damage. If the barbarian is at negative hit points due to lethal damage, she immediately stabilizes.

How exactly does this work?

Is the damage converted prior to the barbarian losing the HP that would set him to 0? Does this happen on a per rage or per attack basis?

To clarify, an example: A level 10 Barbarian is at 1 health. She's hit by an attack and takes 9 damage. This would reduce her to negative 8 health, so Guarded Life kicks in.

So first there seems to be two options:

1st scenario: Barbarian loses 1 health, goes to 0, guarded life kicks in, the 8 damage that would reduce him to -8 are converted to nonleathal and the second half of the Guarded Life ability stabilizes her at 0 hp.

2nd scenario: Barbarian takes a blow that would reduce her to negative hit points, thus before this happens, Guarded Life kicks in, all 9 of the damage taken are converted to nonleathal and the barbarian is left on 1 hp.

Now extend this scenario thus: In the first case, the barbarian is then healed for 1 so she gets back up again and has the required abilities to rage a second time and does so. In the second case the barbarian is still up at 1 hp.

Now she takes another hit for 9. Depending on the scenarios:

First: Barbarian is again reduced to 0 hp, Guarded life kicks in. As it already converted 8 damage, does it now: a) only convert 2 (up to a combined value of 10), or b) it converts the full 8 because either: 1) This is a new rage, so the pool was refreshed or 2) The conversion in each individual instance has a pool of 10.

Second: Barbarian would again be reduced to negative hp, so before that can happen Guarded Life kicks in, this time again there are two options: a) As it converted 9 damage the last time, it now converts only 1 (up to a combined value of 10), the barbarian takes the remaining 8 and is set to -7, then stabilizes, or b) The pool of 10 is hit independent so once again 9 damage are converted and the barbarian is still at 1, with 18 nonleathal damage.

The answer that makes the most sense to me would be First b), 1), but the wording seems somewhat ambiguous.

Answer 1

When your nonlethal damage is greater than or equal to your current hitpoints, you fall unconscious. In the first case, the barbarian drops to 0, and then the remaining damage is converted to nonlethal. The barbarian is stabilized with 0 HP and 8 nonlethal damage.

At this point, if the barbarian receives 1 point of healing, they would have 1 HP and 7 nonlethal damage, and would still be unconscious.

Answering your second question, though... if the barbarian received 8 points of healing, to put them at 8 HP and no nonlethal damage, and then took 15, Guardian Life would convert up to 10 HP of damage into nonlethal again. There's no reason to keep track of previous damage. It's 10 points each time you're reduced below 0 while raging.

Answer 2

RAW what happens is none of your scenarios, but:

3rd scenario: Barbarian takes the blow and is now at -8 HP. Immediately Guarded life triggers (because he is raging and just go below 0 hp) and convert 10 of his lethal damages to non-lethal damages, which brings him to 2 HP and 10 non-lethal damages. As he has now more lethal damages than HP, he falls unconscious.

Anyways, this ability does not have a per-day, per-rage or per-anything limit, it activates each time the barbarian is brought below 0 HP while raging. Technically is would trigger if you are already below 0 HP before the attack, as long as you have a way to keep your rage going (for example with a skald singing or if you are a half-orc).

Of course this becomes weird if you find a way to be immune to non-lethal damage. You shouldn't make this power available for such characters.