Play Dead is a cool ability that can help you to avoid death in PvE. When you activate this spell your character falls and it looks like he is dead. But actually he is just pretending. This spell will help you to survive in a difficult fight when you see that your chances to win are low.
When the ability is active you recover health and mana pretty fast. This allows to reciver your core pools to continue fighting when you are ready. Play Dead can be cancelled any time.