When used, Jigglypuff slowly inflates itself to a very large size, enough to almost entirely cover a small custom stage. While growing, enemies and items will be pushed away. Once Jigglypuff reaches its full size, it shouts "JIGGLY!!!" and shrinks, which causes a massive push that will most likely KO any enemies too close to its body. Jigglypuff then quickly deflates. While Jigglypuff is deflating, it will cause 18% (17% in SSB4) damage and powerful knockback upon contact, similar in power to the push effect. This is in effect until Jigglypuff is back to its original size, making contact dangerous until the Final Smash ends.
While the move covers a very large range, it loses potency quickly on larger stages, and cannot affect anything below Jigglypuff (though this can be remedied by having Jigglypuff start the attack from below the stage, which will let it expand up and through the stage.) Also, it can affect teammates even when Team Attack is off.
Main article: Regenerating terrain glitch
There is a glitch in Super Smash Bros. Brawl where the terrain in some stages (such as Bridge of Eldin) touches Jigglypuff and knocks it out of the move, yet it remains the same size it was when it was forced out of the Final Smash. If Yoshi (or Kirby with Yoshi's ability) uses his Egg Lay on Jigglypuff after this, or Jigglypuff picks up a Super Mushroom, its size doubles, allowing Jigglypuff to become the biggest character in the game. Its size resets when it uses the Final Smash again or gets KO'd. As a pun on its enormous size, the glitch Jigglypuff is often called "Bigglypuff", "Gigapuff" or "Gigalypuff" by fans. When the glitch is performed, Jigglypuff will still have the strength it had when it was at its standard size. Jigglypuff will appear to move very slowly, having the illusion of not being able to jump very high, as well as falling slowly, all attributes equal to when it was at its regular size. As a result of these large hitboxes, should Jigglypuff roll or use its forward smash with the glitch, it will likely self-destruct, as if any hitboxes come in contact with the blast line, Jigglypuff will be KO'd.
Although Puff Up does not seem to originate from the Pokémon games, it does appear to be based on Jigglypuff's other appearances in various media such as its appearance in the anime, where one appears as a giant Pokémon and another would "puff up", which may also be based on its classification as the "Balloon Pokémon". Years later, however, Pokémon Sword and Shield would introduce the concept of Dynamaxing, where a Pokémon can temporarily increase its size and power immensely, although Jigglypuff itself does not have access to Dynamax due to being cut from the game.
In Super Smash Bros. Brawl
In Super Smash Bros. for Wii U
When Jigglypuff uses its Final Smash, it starts to grow bigger. And bigger. And bigger! Before you know it, Jigglypuff will have completely taken over the screen-and there's no way to make it stop. On smaller stages, fighters may find there's nowhere left to stand! At near full size, Jigglypuff will damage and launch anyone it touches.