when you fall sufficiently fast into the ground, there seems to be about a 1 in 3 chance that you fall through the floor.

other details:

hitting something like the rock walls in the gym on your way down stops this from working

seems to be much less likely if you have a lot of horizontal momentum, but much more likely if you have a little instead of none.

most consistent method I've found is dash jumping from the spawn circle in the gym straight down to the Howard area

flight is also a way to reach the sufficient height

i have experienced this while in the gym, hosting park or in matches on ring (both host and client)

i have only tested this in ring, but it almost definitely works in pit too

pushing yourself with a structure on the way down seems to make it less likely (sometimes, very inconsistent)

being in a knockback state while landing seems to make it not happen (unless you are already halfway through the floor when you enter the knockback state)