Bones0
Smash Legend
It's usually referred to as a platform cancel. What happens is when you drop through a platform your character starts out still above the platform (obviously if chars dropped through immediately it'd look like they were teleporting). If you attack someone and it hits before you dip below the platform, you can land instantly. You can replicate this in Training Mode (giving a Starman to the CPU will prevent them from getting knocked away) or V.S. mode by raising the CPU's handicap. Then simply drop through and bair them as early as possible. Not all attacks come out and keep your landing detection high enough for you to land on a platform though. It varies from character to character. Marth can do this with fair, spacies can do this with bair, and Luigi can do it with almost any aerial (because he falls so slow and most of his aerials come out really fast). Furthermore, when you are on a platform that is sinking downwards (PS or FoD as their they transform, the right side of the PS windmill, or the left side of the rotating plats on KJ64) you can do slightly later aerials. Falco normally cannot platform cancel a dair, but when you are on one of these sinking platforms you can drop through and dair and the platform will move low enough during hitlag so that he will be high enough to land on the platform.The other night I was beating up on a CP Fox as Falco and found something I hadn't seen before. When standing next to Fox on a platform, I would drop through and bair instantly, causing me to stay on the platform.
What is the term for this and does it have any value in matches?
As far as applicability, it definitely has some since you are just speeding up a normal situation. It's pretty rare to see people worry about doing it since the time spent falling below the platform usually isn't too much longer, and sometimes you actually will want to followup your move from the bottom level of the stage. If you ever have an opponent on a platform with a laggy recovery, you can do it then to get over to the ledge a little quicker than normal. You can L-cancel it the same as any other aerial, just L-cancel before your character goes into the landing animation.