How does KB work?

I don't quite understand how KB works with bosses.

For example, Pap's KB says 10,000 on HS. So what does that mean? I need to do a minimum of 10,000 damage for him to be knockbacked? That doesn't seem right because when I spam him with upperstab he is not always in the KB animation, therefore still able to attack me. So does KB just work when it wants to?

KB doesn't necessarily mean the monster flinches. You only make it flinch if it's not already in an attack animation. If it is, it'll just slide backwards while attacking.

Once you flinch it, it doesn't stop flinching until you stop KBing it.

Once you flinch it, it doesn't stop flinching until you stop KBing it.

This is what I don't get. Why does it stop flinching even though I'm KBing it all the time? It should be in constant state of flinch as if it were getting pummeled by TT, but it's not. Is it just some skills don't work?

It's just that once it recovers from flinch, it tends to start attacking again (doesn't apply to arans).