I've deleted the previous revisited video in favor of this. Someone mentioned that this attack may be related to how many rads you currently have. I've decided to test it out in Megaton today. Enjoy!
This perk does NOT make you invincible unless the perk is either altered in the future or you're playing on an easier difficulty. Please stop arguing it because I obviously have it and have tested it. You don't hit 0 health, blow up and live. You just die. You MUST be hit between 1 and 20 health.
If you still fail to believe me then take the perk, play on Very Hard and there's your answer.