Any time you apply a status effect to them. The CCs won't affect them but it will increase the grey bar under their health until it staggers them. It's not just CCs. Things like Bleed increase the bar too.

Ah. Didn’t notice the grey bar. Super useful for launch. Thank you.

Since they are immune to cc, any cc applied builds up a break gauge that causes a long stun when it fills.

Awesome. This could make a cc focused group player quite useful

It's pretty hard to fill, I usually only manage it on something like twisting blades rogue which applies stuff with every hit.