Piranha Plant swings its pot around itself.
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Tied with up smash for its fastest smash attack, it is also its weakest, however it is still quite powerful, dealing more damage on the second hit than the first. It launches opponents at a low angle, which when combined with its low hitbox, makes it great for hitting ledge hanging opponents, or for 2-framing.
It can also be used as a read tool, such as reading rolls behind with the back hit, or crossups. It also low-profiles Piranha Plant, giving it strange use as an option to cover landings, which can at times make it better than up smash due to its far better horizontal reach.
Down smash is also Piranha Plant's least laggiest smash attack.
Down smash has certain flaws however. The most notable flaw is that it doesn't hit every ledge hang in the game, meaning it also can't 2-frame every character.
This limits its use at ledge against those who can avoid it simply by hanging onto the ledge, and forces Piranha Plant to go for more riskier options, such as down aerial. It also possesses deceptively low range, as while the hitboxes cover the pot, the trail effect ingame is far larger than the hitboxes.