You shake off the worst of your wounds.[PH:78]
Effect: "You assume the Boundless Endurance stance. Until the stance ends, you have regeneration while you are bloodied. The regeneration equals 2 + your Constitution modifier."
Boundless endurance is a 2nd-level fighter daily utility power. It is a stance that grants the user regeneration while bloodied.[PH:78][Dr398:CCW8]
It is considered a good choice as it is a formidable source of regeneration but competes for a crowded slot.[1]
In detail: This is a stance utility power. Until you leave this stance (resulting from death, KO, encounter ending, or switching to a new stance), you gain the benefit any time you are bloodied. I.e. If Regdar takes 5 points of damage, successfully bloodying him, he gains regeneration +5 (2+3 Con mod). If he goes above 1/2 HP (bloodied), then the effects does not happen. If he is once again taken below 1/2 HP, boundless endurance is triggered.