Selfless Diffusion crossbow skill, how does the cooldown reduction work?

[Toppopia]

I've tried looking it up all last night but its never explained clearly from guides or people how the cooldown reduction works. I see people mention that just selfless diffusion to detonating mark reduces cooldown on skills but i'm not sure i've ever seen it happen in large enough numbers to care about.

I've been second guessing myself recently once i realised that theres a bow skill deadly mark that reduces cooldowns. In dungeons and boss fights I thought that it was selfless diffusion that would reduce my cooldown of annihilation barrage from a minute down to 30 seconds in one skill use, but maybe its been deadly mark this whole time and selfless diffusion is useless except as a remove crowd control skill?

[glaphen]

It just lowers your cooldowns for every weaken you inflict, so if you aren't using the water/fire/wind extra projectiles, you won't get much at all, add on gale specializations.