Can someone explain to me how heavy stun works?

It says that you trigger heavy stun if you hit while primed for stun, with a skill like boneshatter, or ascendancy crushing blows. However, isn't heavy stun also achieved by filling out the bar? So other then for effects like with boneshatter, what does primed for stun actually do if not to trigger heavy stun?

Would it be wrong in thinking of it as culling but for stun? But that makes no sense since you need to hit and skills that generate stun does so much it only takes 1-2 hits to fill out the bar. So even if you hit when primed you overshoot the bar by a mile, that literally anything will trigger a heavy stun regardless.

Does this not make the ascendancy node useless? Why should i get crushing blows when i theoretically heavy stun in a single hit? And no i do not have allot of stun to make this happen. Its a problem from level 1. I have about 100 and just did the first trial. im going to be ending at like 500 so just me looking at bosses is going to stun them. Either this is super broken, or the ascendancy is useless and is nerfing stun builds by removing 2 points from them.

What is primed for stun, and what is the point of crushing blows, and why do we do so much stun that bosses get heavy stunned by just the 2 hits from rolling slam so we can never actually be primed for stun?

primed for stun is really something like "culling" - it allows you to trigger stun earlier, before filling bar

I played as merc, didn't have that much stuns to almost constantly stun bosses, but they do have some kind of CC protection after receiving one, so may be this prime thing can be usefull later on, or for builds that want to achive stuns but with not-so-heavy-hits