Alright. I think some things need to be cleared up.
I was very hopeful for this "discovery" when i first read it, and when i tested it with Pit, i actually got it to "work" however, I think we've been fooling ourselves, and heres why.
What we're seeing, is a mix between IASA frames and the buffering system. What's happening is that some of the characters (maybe all of them i dunno) aerials have IASA frames at the end, and which allows another move (like a smash) to interrupt the lag.
Now first of all, the lag that we are seeing isn't due to fast falling. And I'm going to use pit for these examples. Even if you do a full jump, don't fast fall, but nair right before you hit the ground, Pit will go into his laggy landing animation, and then stand up again. This is due to the mechanic wherein you experience less lag if you complete the aerial before you hit the ground. But Like I said, if you DON'T finish the aerial, then Pit does his laggy landing animation.
Now, the source of the lag behind us, let's continue. Say that there are 15 frames of lag in his laggy landing animation, and the buffer system can pick up an inputted command at frame 5, and execute the action at frame 10 due to the last 5 frames being IASA frames. that's a savings of 5 frames of lag if you buffer a command. What we're seeing with Pit, is when you Nair into the laggy land, and immediately c-stick a smash, nothing happens. But when you go into the laggy land, quickly press UP/X/Y and THEN c-stick a smash, it interrupts the lag and executes the smash. IT LOOKS LIKE AN L-CANCEL, but here is what is actually happening:
You jump with Pit, do his Nair just above the ground (or fast fall one into the ground) and get the laggy land. normally, when you immediately smash the c-stick, you do it within the first 5 frames of the landing, and so it's not entered into the buffer memory, and therefore not executed. HOWEVER, with this "L-Cancel" all that's happening is you are inputting the UP/X/Y command within the first 5 frames, and then when you smash the c-stick, you are at frame 5 or 6, or 8 or w/e, and so as soon as the IASA frames become available, your smash is being executed. here's the proof: this "l-cancel" technique will "work" with any button. If you Nair with Pit into the laggy land, and smash the c-stick twice, you will interrupt the laggy landing with a smash. I'm sorry to say it even works with B--it's all because you aren't canceling the lag with the rapid UP/X/Y, but rather by inputting your smash (or w/e other command) within the range of the buffer capture. You can even cancel the IASA frames with a grab.
Sorry if this seems complicated, but it IS what is actually happening.
disclaimer: the "15 frame" example is hypothetical. it could also be 6 frames of lag total, buffer recognition at frame 3, execution at frame 4, canceling the last 2 frames of lag (IASA frames)