Down air stall
The down air stall (sometimes referred to as WindWaking) is an exploit which allows a character with a stall-then-fall aerial (with the exception of Ivysaur) to float in midair. It was introduced in Super Smash Bros. Brawl, and returned in Super Smash Bros. 4 as well as being initially present in Super Smash Bros. Ultimate, affected Greninja, Bayonetta, Corrin, Simon and Richter in the process, but was removed in patch 4.0.0.
Although it was discovered with Toon Link, the glitch is not exclusive to Toon Link. In fact, it may not be considered a glitch at all. It is an exploitation of the way that Brawl's physics engine works. If a character who has a stall-then-fall aerial performs their down aerial (or in Ivysaur's case, an up aerial) at a certain height they will lag and hover for a while.
How to perform
Using the down aerial repeatedly after being thrown or otherwise launched into the air will eventually cause the character to perform a small hop, then float in the air for a short time in the normal position for their down aerial.