So I was able to investigate this a lot. Apparently there are TWO buffs that need to be activated in order to stop you from getting knocked off:
One is the Stand Firm buff. This is the yellow buff that you get when you activate the Duty Action. Initially, I thought it was wearing off before the hit, but it seems to last unless you move.
The other buff is a blue buff, I believe it's called Braced. I've seen this pop up as soon as you step into the action circle, but looking back I noticed it wasn't always there. I think it has to do with whether you are facing the boss or not. However, I haven't confirmed this yet.
