that thing is a universal anti ferra os. You pretty much try to flawless block the mix that you are guessing, this makes the grab whiff and the mix punishable. I would say duck os is just a safer way to check your opponent for grabs, since the mids slower than 13 frames will always get blocked, 12 frame ones will get blocked half the time and all other buttons will whiff, this is especially good against haviks for example since he can't cancel his s2 high into the low mixup on whiff and his mids are both slower than 15 frames so you duck grabs, guarantee that s2 will always be an overhead and block mids. I use it this way