Reactive Healing

You can channel healing energy in response to an attack that would knock you unconscious.

Prerequisite: Quick Channel or Quicken Spell; channel energy or lay on hands class feature.

Benefit: When the damage from an attack or an effect would reduce you to 0 or fewer hit points, you can expend one use of channel energy (of a form that would heal you) or lay on hands as an immediate action to heal yourself. The healing affects only you, even if it would normally affect others.

Source: Advanced Class Guide