Contrary to popular belief, Poké Floats is not an auto-scrolling stage but rather the camera stays in place as the floats move across the screen. The match takes place on top of giant floating models of Pokémon (hence the name) which are in a constant cycle and whose movement, while deterministic, keeps getting more complex until the cycle is reset. Staying on a float too long can cause the player to lose a stock, as the floats can cross blast lines as they leave the battlefield.
The floats, in order of appearance, are:
After Geodude disappears, the last few Unown clear the screen. Finally, after the last Unown, the cycle restarts (looping back to Squirtle). Squirtle quickly appears from the bottom and the process repeats all over again. A full cycle takes about three minutes and a half.
This stage is not directly based on any Pokémon game, though its "Kanto Skies" designation refers to the Kanto region in which the original Game Boy Pokémon games, and their remakes, are set. It is rather a collection of Pokémon models taken from the Pokémon Stadium series of games.
Interestingly, the Japanese name of the stage, "Pokémon Subspace", does not directly refer to the Pokémon as floats, and the models do not appear to be balloons. The concept of "Poké Floats", added during localization, was later used in other Pokémon media: examples include the WiFi chat rooms in Pokémon HeartGold and SoulSilver Versions, and the climax of the Pokémon: Detective Pikachu movie, which features a battle between Pikachu and Mewtwo on Pokémon balloons.
Poké Floats was originally deemed legal as a counterpick, on the basis that its movement was entirely deterministic and not fast enough to cause an overreliance on mobility, though some characters could have trouble recovering or going from float to float. Eventually, however, the stage was banned from tournament play due to the possibility of stage spikes, and recovery being made difficult by the odd movements and collision detection of some floats, as seen in this clip. It was also deemed as giving a major advantage to Fox because of how easy it was to time out opponents by stalling and the small vertical blast zones, which Fox's KO moves are oriented around.
This glitch can be performed only in version 1.0 of Melee, as a subset of the Name Entry Glitch. If a 0-Second Melee is started on this stage, all floats appear on the screen at the same time before the match ends (image courtesy of Andrewajt62), occasionally freezing the game. This glitch was discovered by Simna ibn Sind at the end of 2013.