From Pippi: Yep, correct. I did it in the MIDDLE of an infinite grab when Marth released. I think you've gotten it wrong though, when you drop out of Marth's grab, there's a LITTLE bit of air time (Yes, this IS ground break) just before Ness hits the ground and THEN the grab break animation. You need to press the jump button ONCE to footstool, as a result, because Marth dropping you out of his grab counts as one jump. More testing is needed, though. I'm pretty sure you can do it on ANY grab that Marth throws at you, infinite or non-infinite, as long as it doesn't throw you too far.
Basically, Marth grabs Ness -> Ness gets grabbed-> Marth pummels->Ness breaks out-> Ness falls-> Ness footstools
NOT Marth grabs Ness -> Ness gets grabbed-> Marth pummels->Ness breaks out-> Ness does break out animation, Ness footstools. Ness doesn't EVEN DO the break out animation. Marth couldn't grab/attack if he tried, because 1. Ness has teleported, 2. He's flinching because Ness is footstooling him.
Also, I did not break out immediately, I did it after a few pummels from Marth. I don't know how to break out right away, sorry...
...I think we have a big one here...
