I can't believe this wasn't discovered in the 1st week of this games existence.
It was discovered a couple months ago. We were just too stupid to realize how Charizard's uair infinite actually worked and that it could be applied to other moves. OOPS!
As to those wanting to learn how to do this, here's a flow I recommend if you STILL don't get it from the videos:
1. Start by learning how to do it with Charizard's uair. Probably easiest to start doing this with. I got it with her (never thought of it, did you? [lolPeanutsreferences,PM me if you don't know the reference]) in a matter of minutes a couple minutes ago (never tried it with Charizard before).
2. Then go and learn Snake's dair ISJR. Probably second easiest.
3. Now to learn how to LEDGEHOP SJR. Take Fox, go on ledge, and ledgehop (AKA, let go of ledge than double jump), then do fair, all as quick as you can do. Then mash jump (or time it if you know how).
4. Platform time! Take Fox, and do it... with his fair. Or try. Yes you can end it at higher heights and still SJR. >.>; You do it by instant double jumping from the ground, fair, then jump when you're about to hit the platform.
5. Now that you get how to do it, do it with whoever the hell you feel like.
The above flow should take a half hour to two hours on average to do, depending on your level of precision. Then just practice it.
P.S. If you do it right, again, there are NO EFFING DUST CLOUDS. Hehehe.
P.S.S. Don't feel pressured to learn it if you don't wanna. It's just a fun thing to do that MAY prove useful. Meh. =P
EDIT: Yes, holding down the jump button only helps multi-jump characters, if it helps at all.