You are especially hard to kill. Not only do your wounds automatically stabilize when grievously injured, but you can remain conscious and continue to act even at death's door.
Benefit: When your hit point total is below 0, but you are not dead, you automatically stabilize. You do not need to make a Constitution check 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 are reduced to negative hit points (even if it isn't your turn). If you do not choose to act as if you were disabled, you immediately fall unconscious.
When using this feat, you are staggered. You can take a move action without further injuring yourself, but if you perform any standard action (or any other action deemed as strenuous, including some swift actions, such as casting a quickened spell) you take 1 point of damage after completing the act. If your negative hit points are equal to or greater than your Constitution score, you immediately die.
Normal: A character without this feat who is reduced to negative hit points is unconscious and dying.
Endzeitgeist Says: If you can't get your hands on the racial ability ferocity, this can save your life - it will only be effective in certain situations, but when it is, you'll love it for it. My group's current barbarian would have died thrice already without this feat - as long as you have this, you're not helpless to enemy attacks and thus can't be finished off via a coup-de-grâce. If you want to be the blood-soaked warrior that defeats his foes and *then* falls unconsciously to the floor - Diehard gives you the tools.