Diehard

This feat will cause you to automatically stabilize when you become incapacitated. This feat is gained automatically by Rangers on level 3.

"Incapacitated" means that you are in the 0/-10 hit points and are bleeding to death. Every tick, a constitution is check is rolled (1d100) to see if you stabilize and start regenerating hit points up to 1. If you reach -10, you die.

If you have Diehard (or you are warforged) you will auto-stabilize, without rolling the constitution check.