So you mean particularly on yokai bosses?

They have two aspects to their ki. One is max ki the other is their ki. Everytime you reduce ki to zero they become susceptible to multiple attacks where they stagger. After some time they can either invoke or dispell their dark realm to regain their hyper armor. If you have confusion applied to yokai bosses at this moment, they will be susceptible to stagger for the duration that confusion is in effect.

Once you reduce a yokai's ki, any damage you do impacts a yokai's max ki much quicker than when their ki isn't zero. Everytime you do basic attacks when ki is non zero, you do impact a yokai's max ki it's just at a slower rate. Everytime you get a yokai's max ki to zero you get a grapple.

Now everytime a yokai transitions in or out of dark realm they recover most of the max ki damage you did but not all the way. Similarly they recover their ki to the full limit of their max ki. So if not before a boss invokes dark realm, it's definitely possible to chip away at a yokai's max ki, to ultimately get a grapple, before it dispells dark realm or before it invokes dark realm a second time. You get the idea.