A staggered creature may take a single move action or standard action each round (but not both, nor can he take full-round actions). A staggered creature can still take free, swift and immediate actions. A creature with nonlethal damage exactly equal to its current hit points gains the staggered condition.
Marius, Scholai Armsmaster says: If you heal yourself while staggered due to being at 0 or fewer hit points, you remove the staggered condition and may continue your round. For this reason, characters with Ferocity or Diehard shine when they have the power to heal themselves through spells, potions, or class features.