I think you're taking about the footstool chaingrab! Popo d-throw, Nana footstool, Popo regrab before the opponent touches the ground (the 'z' input from the grab causes Nana to do a n-air on her way down).
Footstool chaingrab is weird. It's more of a pseudo-infinite. To my knowledge smaller characters can DI the throw and the footstool to get out, but fatties are gonna have a really hard time doing that (but that's less relevant because almost anything we do out of a grab messes fatties up in general; handoffs are an infinite on them and such). And the footstool chaingrab is mashable, since you need to waitt for Nana to catch up to you after the initial grab, then wait for her recover from her n-air in order to start another rep. It's also kinda hard to do (you need to press the taunt button kind of like a shorthop, otherwise Nana will go to high to start another rep without the opponent mashing out easily) and has character-specific timing because it uses d-throw.
That said, it's still super powerful. If they can't mash out of it (which, like a lot of Icies things, is definitely doable below 100%, but the window is brief) then it can rack up damage really well. It's stationary. You can continue it pretty much indefinitely if your timing is good. And it works on pretty much every character. It's a good option if you can get the hang of it.