DISCOVERED A CHINK IN THE PAL VERSION OF THIS GLITCH!!
If you leave a hitbox on the ground and an opponent walks onto it, taking damage, the FIRST hit of the next attack you use will go straight through your opponent, no matter how long you wait after the impact of the dropped box. This means that a sneaky opponent who has been propped up and glitched by your yo-yo can simply DI and fall onto the hitbox, cancelling it out and placing you in a slightly disadvangageous position.
However, a simple tap of A will bring you back into perfect fighting form, and there are a few tricks you can use on a crafty opponent who knows what they're doing.
you can:
A) Hit the opponent before they reach the hitbox. They won't be ready for it.
B) Cancel ghost move with A just after they have hit the dropped box and attack with F-air as they're coming down. (no lag on F-air, so it's very safe.)
C) hit grab just before they reach the ground. With luck, you'll snag them before they hit the box.
So you should be OK if you are one step ahead of the opponent. On another note, an opponent hit by the glitch has only three options:
1: They can fall towards you, in which case you will pwn them with a flying hitbox.
2: They can use their second jump to get in front of both you and the glitch (so you can't get them.) However, in order to do this, they have to use their second jump, which is just asking for trouble, especially against Ness.
3: They can take the hitbox and impare you slightly, which can be countered with methods A, B or C.
EVERY WEAKNESS OF THIS GLITCH CAN BE NEGATED WITH ENOUGH INGENUITY! BEST LEGAL GLITCH IN GAME