Harsh conditions or long exertions do not easily tire you.
Benefit
You gain a +4 bonus on the following checks and saves: Swim checks made to resist nonlethal damage from exhaustion; Constitution checks made to continue running; Constitution checks made to avoid nonlethal damage from a forced march; Constitution checks made to hold your breath; Constitution checks made to avoid nonlethal damage from starvation or thirst; Fortitude saves made to avoid nonlethal damage from hot or cold environments; and Fortitude saves made to resist damage from suffocation.
You may sleep in light or medium armor without becoming fatigued.
When you gain the dying condition, but you are not dead, you automatically stabilize. You do not need to make a Fortitude save each round to avoid losing additional hit points. You may choose to act as if you were disabled, rather than dying. You must make this decision as soon as you incur the dying condition (even if it isn't your turn). If you do not choose to act as if you were disabled, you immediately fall unconscious. You still die when your hit points reach twice your negative Constitution score or lower.
When running, you move five times your normal speed (if wearing medium, light, or no armor and carrying no more than a medium load) or four times your speed (if wearing heavy armor or carrying a heavy load). If you make a jump after a running start (see the Acrobatics skill description), you gain a +4 bonus on your Acrobatics check. While running, you retain your Dexterity bonus to your Defense Value.
If you have a base Fortitude save of +4 or higher, you may attempt once per day as a simple action to make a new saving throw against a harmful condition or affliction requiring a Fortitude save that is affecting you. If this save against the affliction fails, there is no additional effect, but a successful save counts toward curing an affliction such as poison or disease. You cannot use this feat to recover from instantaneous effects, effects that do not allow a saving throw, or effects that do not require a Fortitude save.
Normal
A character without this feat who is reduced to negative hit points equal or lower than his negative Constitution score is unconscious and dying. You move four times your speed while running (if wearing medium, light, or no armor and carrying no more than a medium load) or three times your speed (if wearing heavy armor or carrying a heavy load), and you lose your Dexterity bonus to DV.