Yo-yo glitch

Ness taunting alongside the disconnected yo-yo hitbox, seen in red.

The yo-yo glitch refers to a glitch in Super Smash Bros. Melee done using Ness's yo-yo. It can be used to attack or grab with near limitless range or attach certain single-hitbox attacks to Ness's body. The discovery of the glitch was a collaborative effort between VilNess, LonelyNess, and Dan Smith, and was spurred on by an innocuous video of an unknown Japanese player accidentally dropping a yo-yo hitbox.

The glitch can be performed in all versions of Melee.

Mechanics

Ness's up smash consists of 3 states each with its own hitbox: outreaching, spinning, and swinging. The yo-yo glitch drops the last hitbox of the up smash onto the ground and leaves it there until Ness gets KO'd or uses a non-projectile attack. Opponents can get hit by a dropped yo-yo hitbox or a jacket exactly once. Additionally, if an opponent does get hit by either, then the very next set of non-projectile hitboxes Ness uses also cannot hit that opponent (first hit of a multi-hit attack, such as forward aerial, will whiff that opponent, but the following hits can still connect).

How To Perform

Ness's up smash must first hit any opponent or damageable object while in its outreaching or spinning state. The outreaching state is typically used to hit, as its hitbox is stronger than the one in spinning state, and can knock the victim out of the way. If only outreaching state hits, then Ness has to charge his up smash for 6 - 11 frames after the hit. If the up smash is unstale, that equates to attack button being released between 24th to 29th frame of the up smash (including hitlag frames). If spinning state hits, the up smash has to be charged for 7 - 12 frames. The yo-yo can't hit anything after it is released into its swinging state.

This glitch works with down smash accordingly but requires that said down smash attack be canceled in its ending frames by an attack or effect (such as a Mushroom).

Followups

Deadline: If Ness uses any non-projectile attack (including a grab) after performing the yo-yo glitch, any characters or damageable stage elements between Ness and the location where the glitch was performed will be hit by said attack. If a grab is used, then only the target closest to Ness will be affected. This followup technique to the yo-yo glitch was discovered by LonelyNess.
Thunder Jacket: After performing the yo-yo glitch, if the player blasts with PK Thunder 2 against a floor or a grabbable ledge, the hitbox of the move will be attached to Ness' body. Besides hitting opponents and destructible stage elements, it can also clash with opposing attacks. This followup technique to the yo-yo glitch was discovered by Simna ibn Sind.
Spike Jacket: If the player uses Ness' down aerial and lands immediately after its hitbox comes out (which can be done by DJCing or SHFFLing), it will be attached to his lower body, meteor smashing opponents on contact. This followup technique to the yo-yo glitch was discovered by MasterofFlames.
Star Jacket: Similar to the Spike Jacket, the hitbox of Ness' up aerial will be stuck to his upper body if he uses the move and lands after the hitbox comes out. It was also discovered by MasterofFlames.
Toy Jacket: After performing the glitch, if Ness uses an up smash or down smash on a disappearing platform (such as the awnings in Onett) and times it so that the platform disappears right as the yo-yo comes out, the hitbox of either move will be attached to his feet, causing either weak or strong yo-yo knockback based on the timing of the setup. This followup technique was discovered by Earthbound360.
Kirby Jacket: If Ness is Inhaled and spat out by Kirby after performing the glitch, the hitbox of Kirby's star launch will be attached to Ness' lower body. This followup was discovered by Fisac.
Throw Jacket: If any other character throws Ness into a wall or ceiling after he performs the glitch, the hitbox of said throw will be attached to Ness body, damaging opponents on contact. Interestingly, KOing them in this way yields the Dead-Weight KO bonus. This followup was discovered by Kahnu.

Jackets

After using the yoyo glitch, certain other attacks can be used to replace the yo-yo hitbox with the hitbox of the attack used. The rules of the regular yo-yo glitch still apply when using a jacket.

PK Thunder - must either be used to hit Ness into the ground or at a ledge to grab it.
Down aerial
Up aerial
Down smash
Inhale
Throw - must be thrown at wall or ceiling
Parasol
Barrel Cannon

Explanation

Hitting something with the Yo-yo creates a special delayed hitbox refresh event. The purpose of the hitbox refresh is to allow a charged smash attack to hit a single target multiple times, while the Yo-yo is spinning on ground. However, the refresh event is missing an important detail that normal hitbox creation events always have: it doesn't setup Ness to have his hitboxes removed when his state changes. Thus, the refreshed Yo-yo hitbox can stay active after the smash attack has already ended. ^[1]

Down smash yo-yo glitch

It is also possible to do yo-yo glitch with down smash. However, there is very little to no information about it. VilNess has succeeded doing it twice with making a CPU Marth recover with upB over the ledge and fall down to grab the ledge while some hitbox of the dsmash yo+yo starts to fall under the ledge when charging.

Gallery

Ness performs the first step, creating a trap with a dropped hitbox.
If the enemy is in between Ness and the dropped hitbox, all of his standard attacks, smash attacks, grabs, and PK Thunder 2 will hit the opponent.
Not only can one grab the enemy, but smash attacks work too.
Using the deadline to smash from long range, with hitboxes visible.
Deadline even works with PK Thunder 2, the attack that occurs when Ness hits himself with PK Thunder.
Ness runs into Samus while wearing a Thunder Jacket
The glitch is viable for tournament use: here, Mofo performs the glitch and grabs Hax (Captain Falcon) from long range.
In the same match, Mofo installs a Thunder Jacket. He is hit by Hax, who is KO'd by the jacket. Mofo is hit by the tail of his PK Thunder and survives.
Toasty turns the hitbox into a down aerial hitbox, and then charges his PK Flash. The down air hitbox pops Link into the air, where he is hit by the PK Flash.