Question about "Pluck of Many"

In a couple videos I've seen now on this subreddit and YT, I've seen clips where the user does the 4 Focus heavy for Smash Stance, and all of their duplicates do the same attack at the same any. I can't find this ability in the skill set, is it a specific armour that does it or something else?

I think it'd really help me, I've just beaten "Father of Stones" in Chapter 5, for reference of where I am in the story.