Peach players usually refer to that tech as JCFF. (Jump Cancel Fast Fall)
Basically, you are cancelling both your jump and float. But you are, at the same time, cancelling your jump
using your float.
From EOE:
“This can be performed by pressing and holding down the jump button, then holding down on the control stick and instantly releasing the jump button when you want to fast fall.”
Here’s a link for this exact topic:
You’ll notice it’s Smash WiiU but the same method applies to Ultimate.
The timing takes some getting use to but once you get it down it’ll become a very natural way of moving the character around. This tech specifically is what makes Peach so fun for me to play. It probably goes without saying but Loud Yellow Peach with Brown Hair
can do it too.