Bloodletter and Rain of Death are oGCDs—off-global cooldowns. Heavy Shot and Quick Nock are GCDs—skills that are on the global cooldown. You are able to weave in an oGCD between two GCDs, thereby gaining the benefit of both without interrupting your GCD.

So, for a single target, you would weave Bloodletter between Heavy Shot uses: Heavy Shot > Bloodletter weave > Heavy Shot > Heavy Shot > etc.. Repeat each time Bloodletter comes off cooldown or is reset during Mage’s Ballad.

For multiple targets, you would weave Rain of Death between Quick Nock uses: Quick Nock > Rain of Death weave > Quick Nock > Quick Nock > etc.. Repeat each time Rain of Death comes off cooldown or is reset during Mage’s Ballad.

Under no condition should you not use Bloodletter or Rain of Death. They should be used with your GCD skills to push out more damage. There will be other oGCDs introduced to BRD: you will use them in a similar fashion: woven between GCDs. You should be able to comfortably weave up to 2 oGCDs between GCDs.