Hellfire Blast (Sp) [...]
Each time you use this ability, you take 1 point of Constitution damage. Because the diabolical forces behind the power of hellfire demand part of your essence in exchange for this granted power, if you do not have a Constitution score or are somehow immune to Constitution damage, you cannot use this ability.
(Fiendish Codex II: Tyrants of the Nine Hells, p.90)
A heavy web belt of cyan energy wraps around your torso. When you wear it, you feel energized and revitalized. When you are struck by an attack that would damage your ability scores, a wave of incarnum energy passes through you, blunting the effectiveness of the attack.
The strongheart vest protects you from attacks that would reduce your ability scores. Any time you would take ability damage, such as Constitution damage or Strength damage, the amount of the damage is reduced by 1 point, to a minimum of 0.
For example, if you fail a saving throw against the poison of a monstrous centipede and would normally take 2 points of Dexterity damage from its poison, you take 1 point instead. Ten rounds later, if you fail a second saving throw against the poison and would normally take 1 point of Dexterity damage, you take no Dexterity damage instead.
(Magic of Incarnum, p.89)
Question separated from Can at-wills be used to fuel other abilities that require spending uses?