Source: Player's Handbook
You can remain conscious after attacks that would fell others.
Benefit: When reduced to between -1 and -9 hit points, you automatically become stable. You don't have to roll d% to see if you lose 1 hit point each round.
When reduced to negative hit points, you may choose to act as 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 can take either a single move or standard action each turn, but not both, and you cannot take a full-round action. You can take a move action without further injuring yourself, but if you perform any standard action (or any other action the DM deems as strenuous, including some free actions, such as casting a quickened spell) you take 1 point of damage after completing the act. If you reach -10 hit points, you immediately die.
Normal: A character without this feat who is reduced to between -1 and -9 hit points is unconscious and dying, as described in Dying.