While this skill refers to damage being fatal as the trigger, it actually triggers whenever health would reach zero, through damage, degeneration, life steal, life loss, or life sacrifice.
Using this spell right after a mid battle resurrection can help you get back to healing while not worrying right away about the person just resurrected being ganked.
The description is somewhat ambiguous. Although it states the damage is negated and then healed for the stated health, it actually sets the character's health to the stated amount. Thus, a creature with sufficient damage to go over ~240 in one hit would still lower the target's health overall.