The tooltip says if your are under a Fear, Stun or Charm effect, all damage taken is reduced by 75%.
I'm not sure what effects are counted as fear, stun or charm. Some people seem to think it covers all crowd control effects, but I'm doubtful. For instance, does frozen or imprisonment count? When Diablo puts you in the cage and then grasps you in his hand, does it count? I'd like a complete list of what counts as a Fear, Stun or Charm effect.