Characters with multi jumps can do it. Charizard's uair infinite is this technique. If it works the same for other characters with multi jumps as it does for Charizard, then... daaamn, multi-jump characters will just own. XP I think how it works is it thinks you're on the ground but not, so it resets your double jumps and uses one for this technique. AKA, if MK does it and gets hit while trying to continue it, he'll still have four double jumps...
...daaang.
EDIT: And I think the timing will get really easy when we get used to it. <.<; I love how Yoshi can bair up once, then down as he starts falling, then SJR, than rinse and repeat. I LOVE IT! I also love how he can air dodge on the way up, or even nair, than do another aerial on the way down than SJR. Love it.