You are on 100% HP.
You take a hit for 500.000 damage. You don't die, you're still at 100% hp but you gain a debuff that absorbs 500.000 healing. If you don't get healed for 500.000 in the next 3 seconds you die.
This makes it less desirable in PvE compared to Cauterize, because you can't soak big-hitting boss mechanics all by yourself. It's a good talent, but the 3-second window makes it somewhat situational and limited in use.