Once a fighter arrives at the apex of an airborne period, tapping down on the control stick makes them fast-fall, returning to earth much faster and allowing long-lasting air moves to be pulled down along a tall vertical swath. This doesn’t just work during jumps; you can also fast-fall after being launched! The only time you can’t fast-fall in midair is during special-move execution, or air grabs (“zairs”).
This is an extremely important and subtle movement skill to develop. It’s almost easier to talk about when you wouldn’t want to fast-fall than when you would. When leaping aggressively at foes and sticking out air pokes, you’re pretty much always best served by fast-falling to cut short the second half of the jump, and to maximize the amount of airspace that your air poke hits. Hopping forward with air pokes, then fast-falling is so common and important in Smash that there’s an acronym you might see used: SHFF, for “short hop fast fall.”
Defensively, when jumping and sticking out air pokes to wall off aggressors, you might forgo fast-falling so you have time to squeeze out an extra air attack or two. When you’re tumbling after being launched or thrown, sometimes your attacker will count on you to fast-fall, eager to get back to land faster, and aim their juggle attempts accordingly. Drifting wide away from them and skipping the fast fall can mess up their combo attempt. And offstage, when you’re trying to recover, it’s obviously not the time for fast-falling.
Use the C-Stick to Improve Fast Fall Control
When using the control stick plus the Attack button for air moves, you always trigger a fast fall while attempting down air attacks. To down air without fast-falling built in, flick the C-stick downward instead. Getting used to using the C-stick for air moves in general helps solve some aerial execution challenges like this. For instance, when trying to “SHFF” aggressively with forward air attack, you’d need to tap the Jump button (or up on the control stick, if you leave “tap up to jump” enabled in extra control options), tap forward+Attack, then tap down. It’s hard to always be totally crisp with control stick motions under duress, so it’s easy to mess up part of this; you get a neutral air attack instead of forward air attack, or you tap down too early or too unassertively, failing to trigger fast fall. Instead, try hopping, flicking the C-stick forward to trigger forward air attack, then tapping down on the control stick for the fast fall. You might find this is more consistent and easier to do rapidly; it might not sound like much, but it makes a big difference, not having to worry about getting the control stick into the forward position repeatedly for forward air attacks (or whatever).