All I know is
Up-Smash real quickly
I figured out a trick but I don't know what to call it. Jump cancel? Fall cancel? Attack cancel?
First, you need to stand on a platform. Moving ones like Smashville's work best.
Next, drop down by flicking the control pad down.
Now you can press any command. Timing is crucial: you must still be on the platform but no longer standing on it.
I use Jab (Neutral A) since that seems to be the easiest. If done correctly, the character will fall and then appear back on the platform without jumping.
Smashes are very useful when performing this. However, they have the hardest-to-see results. Try in slow motion to somewhat see it clearly.
If you're good at it, you can move your character around the platform quickly, and use tilts as well. This works with all characters.
EDIT: Most of these won't damage the opponent, but some (like Smashes) will. It works when landing on a platform too.
Way t' rediscover an AT. I did the same way back when actually, accidentally discovered glide tossing a decent amount'a time after it was known. I used glide tossing in my early With Anyone days without realizing it was a legitimate AT. Someone on here pointed out the AT after we were goofin' off with items on... so then I Googled it.
And that's how I inadvertently learned 'bout DACUSing.
... Ninja, if you're takin' his words as the wisdom of a pro, you're sorely mistaken. No offense Trago. >_>;
Perhaps I'm readin' it wrong, but that's not exactly the most useful way t' use Platform Canceling neither (I'm not even sure how that is useful actually, explain t' me in detail, yeah?). You're s'posed t' use PCing to cancel your jump and immediately attack, cancel the jump and instantly drop through the platform or simply canceling the jump and dashing/fast walking as an approach option. Characters with really floaty full jumps or high jumps like Falco can especially make use of it as an approach option at the platform's height since it's ridiculously quicker than doin' a full jump and then landing on said platform via fast fall.
That and Marth's really use it a lot.
If I remember correctly, you fall through the platform, press jump, and then flick the direction stick down, and then perform the attack.
Also can be done by approaching the platform from below.
also it can only be performed on a moving platform