The World of Valden
Heroic (or villainous) characters are distinguished by their ability to go above and beyond the limits of a normal creature. They have reserves.
When you rest, any excess healing gained through sleep is kept as Reserves. Reserves can be used to return a character to 0 HP when a blow would normally kill or knock them unconscious. Reserves do not stack, they always overwrite even if lower than before. A character with diehard can choose to use this to reach one above his death threshold, including when the damage from Dieharding would kill them.
Reserves cannot be used unless you use/have enough to leave you conscious after a blow. However, if you have at least 1 point of Reserve in your pool while unconscious, you gain a +2 bonus to stabilization checks.