DRK Defensive CD's. Am I missing something?

[CapricaLangley]

Yea I was wondering about this as well. Glad people explained it.

BTW how exactly does TBN work?

It gives you a shield, but remember that your aim is to make enemies pop it, in order to get resources back and buff oGCD. You should use it on large packs or tankbusters, not for standard mitigations, because you need to take roughly 30k damage to make it pop. TBN is the reason you should never reach less than 3k MP if you don't know the encounter mechanics, it's a good MP management exercise.