In very quick succession:
• Drop through the platform (hold down).
• Do a nair (return the stick to neutral and press the A button).
• L-cancel (Press the L, R, or Z button). Technically this is optional.
You have to hit the opponent for this to work.
Assuming a normal platform drop from a standing animation, the frame data is:
(Inputs take effect on the frame after they are made)
Frame 1: Standing; press down on the control stick beyond the threshold to drop through the platform.
Frame 2: Crouch animation ("squat"); you have to keep holding down here or you won't fall through.
Frame 3: Crouch animation ("squat"); you can stop holding down now and you'll still drop through (meaning you have to input down for at least frames 1 and 2).
Frame 4: Crouch animation ("squat").
Frame 5: Platform drop animation ("pass"); you will land on the platform if you input the nair here.
Frame 6: Platform drop animation ("pass"); you will land on the platform if you input the nair here.
Frame >6: Platform drop animation ("pass"); you will keep falling if you input the nair here.
Therefore: you have to hold the control stick down on at least 2 consecutive frames to drop, you have about 4 frames after pressing down on the control stick to release it in time to perform the nair, and you have 2 frames on which you can input the nair itself.
It can be hard to do; you can perform the inputs with (what feels like) the same timing over and over but mess up anyway. The speed and repetitive nature of the inputs can lead to you getting tired out quickly; don't practice for too long in one session.
Some things that can go wrong:
• Not holding down the control stick for long enough to drop.
• Continuing to hold the control stick down when the A button is pressed (results in a down tilt or down smash).
• Pressing the A button too late (results in a nair falling through the platform).
• Pressing the A button before moving the control stick down enough to register anything (results in a jab).
• Pressing the A button before releasing the control stick after pressing it down (results in a dair falling through the platform).
• Pressing the A button after letting the control stick return to neutral after it has been tilted down, but not enough to make Luigi fall through the platform (results in a jab).
• Hard pressing the L or R button for the L-cancel before the aerial (results in an airdodge).
• Performing the correct inputs but before the opponent is in range (results in a nair falling through the platform).
For a while my main problem with inputting this was not holding down on the control stick long enough (I didn't realise that you weren't supposed to flick it as quickly as possible until I looked at the frame data).
If you can, practice using something that shows whether you've hit an L-cancel (a Gecko code or the 20XX pack, for example), since it's difficult to tell if you're doing that part right.
If you use handicaps (set yourself to handicap 1 and the opponent to handicap 8; due to a bug this is more effective than handicap 9) then you can practice more efficiently since your opponent will not get sent as high and will survive to much higher percentages. Just make sure they're using a controller port that respawns above a platform, since you won't be able to get them onto one from the ground without building a lot of damage first. The 20XX pack save states are also very useful, since you can reset yourself and your opponent easily.
