Cooldown of 15 seconds.
And that it does allow healer to get a breath of air.
Its effectively 25% HP more which scales with mitigation we have.
30k hp which will by increased by 20% by a trait is 36k.
Now how much do you need to receive in damage in order to match this number with standard mitigation tool?
Lets say its 20%, you will have to take 150% of your total HP in order for your mitigation tool to match the number of effective health TBN provides for DRK.
You will have to literally get yourself killed 1.5 times so the 20% of mitigation you have will be able to mitigate 36K from enemies, and i do not even count additional 20% into equation from a trait because it would make even a bigger number, you will have to take 225k damage, 187.5% of your HP in order to match TBN effective health.
This what makes TBN so powerful tool to play with, it is upfront HP mitigation which scales really well with other % mitigation tools.
You give your healer a chance to dish out 1 or two DPS moves during TBN, IT IS A DPS GAIN but its not your personal dps, but a team dps.