Fastfall

“Pressing down on the analog stick after the apex of your jump (i.e. while falling) will cause your character to fall much faster than usual. Your character will continue to fast fall until either landing on the ground/platform or taking a hit.”
— melee.guru

Fastfalling attacks can be a mixup game. You can dodge anti-air moves like up-tilts by not fastfalling when an opponent expects it, or vice versa. Most of the time, though, you'll want to fastfall, since it leaves you actionable quicker.

You probably know that you can fall through platforms while fastfalling. But did you know that this mechanic actually has nothing to do with fastfalling? In fact, you can still fall through platforms without fastfalling as long as the analog stick is held down.

This is not often used; if you're attacking through a platform, you probably want to fastfall or maybe Shield drop as a mixup. But it is an option.