Soundwaves and music notes come out from Jigglypuff's body and travel out a certain distance. All opponents within the range that are touching the ground will fall asleep for a short amount of time. This attack does no damage to playable characters and simply disables opponents to set up for attacks. However, as enemies and bosses cannot be put to sleep, Sing will deal damage to them instead. Similar to a grab, opponents can escape the sleep state more quickly by button mashing or rapidly moving the control stick or d-pad (except in the original Super Smash Bros).
In Melee, Brawl and SSB4, the more damage the opponent has, the longer they will remain asleep. However, in Super Smash Bros., the opponent remains asleep for a longer time if they bear a low damage. The time the opponent remains asleep is shortest when bearing any damage over 300%. Additionally, in SSB4 a sweetspot was added closer to Jigglypuff which puts opponents to sleep for longer than usual, with the third soundwave only featuring said sweetspot, and no sourspot. The sourspot's sleep duration is the same as the entire hitbox's sleep duration in Brawl.
Unlike other up special moves, Sing gains no height and thus is not a recovery move, making it a very unusual up special move. Instead, players can often use Rollout or an air dodge as an extra jump, or simply use Pound for horizontal recovery. However, it does increase the size of Jigglypuff's ledge-grab box.
Timing of this move is crucial, as the duration of the sleep is short - meaning that opponents who fall asleep at the beginning may wake up before Jigglypuff finishes singing if they are at a low damage percent. This will leave Jigglypuff very vulnerable to counter-attacks. Using the move in midair can shift this risk somewhat, by ensuring that opponents who fall asleep will do so closer to the middle of the move than at the beginning (so that they will not wake up before the move is over); however, this does not change the fact that the move has no effect on midair foes, and so Jigglypuff will still be vulnerable, only at the beginning of the move rather than at the end.
From Melee onward, Sing can be ledge-canceled, which greatly improves the utility of the move. If Sing is used when Jigglypuff is in midair over a ledge and an opponent is nearby, then the opponent will fall asleep at the beginning of the move, but then Jigglypuff will grab the ledge and the rest of the move will be canceled while the opponent is still asleep for the usual duration. This can be used to set up combos, and can even stall indefinitely with the correct timing.
The move is very rarely used in competitive play for singles, due to the move possessing three major flaws. Firstly, the move has very long startup lag with a huge visual cue (since the first soundwave appears before the hitbox comes out), and due to the move dealing no damage to shields and having long ending lag, the move can very easily be shielded and punished. Secondly, the move's range is very limited, especially considering that the move only affects grounded opponents, and as such, against a good opponent, the only realistic way to land the move is to catch an opponent during the ending lag of a powerful attack. However, the move's long startup means that it is often more effective to punish the ending lag with Rest instead, which throughout the games has, at most, 1 frame of the startup. Finally, although the move usually puts the opponent to sleep for slightly longer than the move's duration at mid to high percents, the opponent can easily mash to wake up before Jigglypuff can act again, making the move nearly useless in singles. However, Sing can be used in team battles very effectively by having Jigglypuff put enemies to sleep, then having its teammate use a fully charged smash attack or powerful special attack like Falcon Punch on the sleeping opponents.
In Ultimate, Sing has been significantly improved to the point where it finally has some utility in competitive play, even in singles. While it's still extremely punishable, its range has been notably increased, and now the opponent is put to sleep for long enough for Jigglypuff to be able to reliably punish them after landing it, making it an easy and guaranteed KO set up at 50-70% if Rest is used. In addition, Jigglypuff can now slightly drift in the air when using Sing. However, it's still slow, has a clear visual clue, and can be punished very easily as the move can still be blocked by just shielding. Because of this, Sing's utility is limited to hard reads, such as punishing a predicted roll.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Sing is based off a move of the same name in the Pokémon games. Introduced in Generation I, Sing is a Normal-type status move that has the sole effect of putting the opponent to sleep. For some Pokémon like Jigglypuff and Blissey, this is the only move they can learn that puts the opponent to sleep, but Sing's very poor accuracy rate of 55% makes it unreliable.
In every Pokémon generation, Jigglypuff's evolutionary family is well known for using this move. A particular Jigglypuff in the Pokémon anime is a recurring character of the early seasons who wants to be applauded for its music but inadvertently puts people to sleep instead; in fact, the sound clip of Sing throughout the games is a combination of its game version and its anime version, as Jigglypuff intones the song with the same first syllables as in the anime.
Hyper Voice, included in Super Smash Bros. 4 as one of Sing's custom variants, is a Normal-type attack introduced in Generation III that can hit multiple opponents. Jigglypuff can also learn this move by leveling up.