Obscure mechanics: FCO Drift and Soft-hopping.
First, let me point out that, while I have no idea who originally discovered soft-hopping, I'm almost certain it wasn't me. However, I do believe I am the first person to discover FCO Drift.
FCO Drift (freefall carryover drift) is an obscure glitch/mechanic, which can potentially be detrimental to those who are unaware of it. As you probably know, during special fall (freefall) most fighters' air speeds (the fastest they can drift through the air) are noticeably lower than usual. As it turns out, this has an unusual effect on some fighters. If it's possible for a fighter to land after using their up special, before they enter freefall, and the animation for the up special does not continue upon landing, they will temporarily be placed in a glitched state. What this means is the next time they enter the air, either by jumping or running off an edge, their air speed will drop down to their freefall air speed after a few frames, and thus they will drift through the air slower than usual. This glitched state will persist until they land, grab a ledge or double jump. For many fighters, this will almost never trigger, but on Battlefield, Pikachu and Captain Falcon are particularly vulnerable to this. Here's a video I recorded a few weeks ago documenting it:
(Was just going to post just the video, but SB won't let me do that...)
Soft-hopping is a technique which is sort of similar to IDJ, but with different activation and applications. Technically, it's existed as early as Brawl, but had virtually no functionality prior to Ultimate. This is because prior to Ultimate, the only thing it did was allow you to bypass jumpsquat, but now, it allows you to perform a slower, floatier full hop, akin to full hops prior to Ultimate.
You can perform a soft-hop by running off an edge, any edge, and jumping immediately after doing so (there's a window of roughly 4 frames). And that's it. However, since this gives you a floatier full hop, it allows you to do 2 important things: firstly, it allows you to use aerials lower to the ground (or platform) without having to shorthop, which is also something that IDJ can be used for, and it allows you to get further out offstage without having to burn your double jump, due to increasing your air time.
Compared to IDJ, it won't burn up your double jump, so you don't have to fastfall and land to extend a combo, but it's more situational, since you have to run off an edge in order to use it. To be honest, I have no idea how useful this is, as I'm not a labber, and again, hardly anyone knows about this technique. Once again, here is a video in which I showcase it, which I posted earlier today:
I apologise for my terrible explanation skills, but I think the videos do a good job at showing the basic gist of what happens.