Helpless

Helpless (also known as freefall, special fall and various other names, and called "fallspecial" in game data) is a state in which characters are unable to take any action, with the exception of grabbing a ledge, manoeuvring left/right, fastfalling, or climbing a ladder, until they land on the ground or pass a blast line. Helplessness is typically associated with just having used one's recovery move(s), namely the up special, though some other special moves that provide additional movement also incur this state. Before Ultimate, many special moves involving controllable projectiles (such as Ness's PK Flash and Zelda's Din's Fire) caused helplessness as well.

Some special moves are programmed to transition to helplessness early upon contact with a platform. In the case of soft platforms, holding down to drop through them will result in the player becoming helpless before the move is usually over, though it can also be seen if the user breaks the platform it landed on with the landing potion of the attack (where applicable), or if blown off the platform with a move that deals no hitstun.

Helplessness most commonly ends upon landing on the ground or being KO'd, although it also ends if characters flinch, allowing them to attack or re-use their triple jump. A helpless character is denoted by both a flickering black overlay and a distinct falling animation.

A state similar to helplessness exists in falling after having one's shield broken and then being stunned.

Curiously, the helpless state gives characters an extra ability in Super Smash Bros.: the ability to fall through soft platforms without landing on them. Starting in Melee, this can be done at any time (not just when a character is helpless).

Landfallspecial and "superdashtechnique"
As a unique animation is used for falling while helpless, characters enter a different animation than usual once they land on the ground. This is called "landfallspecial" internally in Melee. Because air dodging enters this state and wavelanding involves air dodging into the ground, when wavedashing is performed, Melee's shows "landfallspecial" as the character state. This originally led some players to believe that the "landfallspecial" designation was specifically designed for wavedashing and that developers intentionally put wavedashing in the game, further fueling the various debates over wavedashing's legitimacy. At one point, the myth morphed to where, instead of just "landfallspecial", there was purportedly another debug mode designation — cited as "superdashtechnique", "superdashattack", or other supposed variants — created solely for the wavedashing animation. All incarnations of this story have been proven false, and should not be themselves taken as evidence that wavedashing was intentionally put or left in Melee.

Freefall Carryover Drift
In Ultimate, if a fighter uses a special move which will put them into freefall, but they land before entering freefall, they may enter a glitched state if the special move's ending lag doesn't transfer from an airborne to a grounded state. Upon entering this state, the next time the character enters the air, they will initially adopt their normal air speed, but after a few frames will revert to the air speed they would have when entering freefall from the special move used, which is usually lower than their normal fall speed. This will persist until they double jump, grab a ledge or land on the ground again.

Trivia

 * The moves Fire, Robo Burner, Spring Jump, and Cypher do not cause helplessness in Brawl, although they do restrict the user's choice of moves until they land - Mr. Game & Watch and Snake simply cannot use the recovery move again, while Sonic cannot use any special moves and R.O.B. cannot air dodge. Through a programming oversight, this condition prevents them from using their recovery move again if grabbed and released without pummeling before touching the ground (except for R.O.B.).
 * If Kirby copies a move that causes helplessness, he will be affected just as the original character would be. These moves are Giant Punch (Melee through Smash 4), Mach Tornado, PK Flash (until Ultimate), PK Freeze (until Ultimate), and Rollout (only Melee).
 * In all games except Melee, some characters are unable to become helpless through their own moveset. These include Yoshi, Jigglypuff, Mr. Game & Watch, Snake, Ivysaur (in Ultimate), Sonic, R.O.B., Mega Man, Bowser Jr., Bayonetta, Banjo & Kazooie, Byleth, Min Min, Pyra and Kazuya.
 * R.O.B. is not programmed to have a helpless state in Brawl, and attempting to see it via hacking causes the game to crash. All other characters in the game have a helpless state, even if they cannot enter it without hacking.
 * Moves like Final Cutter, Stone Scabbard and Prominence Revolt do put their respective characters into a helpless state, but only when landing on the ground, making this mechanic unnoticable in normal gameplay. If the character is somehow pushed off the stage before exiting landing lag, they will retain their helpless state and fall to their deaths.
 * Smash 4 and Ultimate introduced Pac-Man's Pac-Jump, which automatically triggers the helpless state to characters that touch the red trampoline. This is the only way to see the helpless animations of the aforementioned characters in normal gameplay.
 * In Brawl, since Jigglypuff's and Mr. Game & Watch's air dodge animations carry over from Melee, pausing just before the animation ends allows the player to see their helpless state.
 * Additionally, whenever Link, Samus, and Toon Link perform a grab aerial, it's possible to see their helpless animations by pausing just before the end of the animations.
 * Meta Knight is the only character whose entire set of special moves leaves him helpless.
 * Meta Knight is also the only character with two different helpless animations; one with his wings, and one without.
 * Donkey Kong is the only character to have a move not result in a helpless state in the first game (being Smash 64), then having it result in a helpless state for the next games (from Melee to Super Smash Bros. 4), then reverting back to not resulting in such (being Ultimate). In this case, Giant Punch.
 * The unique animation caused by landing while helpless is the cause of the landing lag glitch.
 * Melee is the only Smash game in which all characters can become helpless through their own moveset.
 * Every game in the series except Smash 64 and Brawl has a method in which a character can become helpless as a result of someone else:
 * Melee has the, which can be thrown at someone to make them perform a move similar to Samus's , leaving them helpless after it.
 * Smash 4 and Ultimate have Pac-Man's trampoline, which will make a character helpless if they bounce on it while it is red.