Double jump cancel
The double jump cancel, sometimes abbreviated as DJC, is a glitch-tactic used by Ness, Mewtwo, Peach, and Yoshi, who can attack immediately after their double jump to cancel their upward momentum and land on the ground soon after their attack. This tactic is used to perform aerial attacks incredibly quickly, thus allowing aerial combos and KO's to be performed more quickly. Double jump canceling has almost been completely removed in Super Smash Bros. Brawl, some specials are the only moves that can cancel character's midair jumps.
Ness
Ness' play in Smash 64 is almost completely reliant on this technique. Because his aerials provide high hitstun and damage, as well as having large combo ability (especially when coupled with other moves such as his up tilt), he can perform zero-death combos on several characters. At one point this ability was even considered broken. However, it has since lost such recognition due to Ness' short range, but nonetheless is an important part of his metagame and should still be commonly used in high level Ness play.
In Melee, Ness can still use the technique, but it is not as useful due to the nerfs he suffered in the game.
In Brawl, Ness can no longer perform double jump cancels using his aerials, though his special moves can be DJC'd. He can still pull off some techniques with DJC'd PK Fire, such as PK Jump.
Peach
For Peach, this tactic is usable, but generally unused due to her slower aerials and superior float cancels. The pro Peach user XIF is known for using multiple DJC'd neutral aerials.
Yoshi
Yoshi's double jump cancel is extremely useful in Smash 64, allowing him to pull off surprisingly fast down aerials, and with the attack doing up to 56 damage, he can even get a 0-death combo with some characters.
For Yoshi, he can use this to "counter" attacks because of his double jump's unique properties.
Yoshi can perform this technique in Melee as well, though it is less effective, albeit still potent and useful in his metagame.
One obvious downfall of this tactic in both games is that Yoshi loses his double jump, possibly resulting in a self-destruct when away from an edge. This can be extremely detrimental to him, as he lacks a third jump, and if Yoshi is hit away from the stage, Yoshi will have almost no means of recovery, outside of the directional air dodge in Melee.
Like Ness, Yoshi cannot DJC normal aerials in Brawl, but he can use DJC'd special moves.
Mewtwo
Mewtwo gains a significant advantage over other characters in that he can utilize the DJC. The best use of the DJC for Mewtwo is speeding up his forward aerial, though it can be used to assists his up aerial and back aerial as well (neutral aerial and down aerial take too long to be truly helped by this technique). As with all characters, Mewtwo must not perform the DJC frame perfect, as he will hit the ground before his air attack connects.
DJC physics
Since the affected characters' double jumps are of the "delayed" type (where they do not rise immediately), interrupting the action with an attack will cause the scheduled upwards force to not take place.
A similar mechanic can cause superjumps.
Double jump cancel fast fall L-cancel
Double jump cancel fast fall L-cancel (abbreviated as "DJCFFL") is a tactic that is simply a quicker application of the double jump cancel, and is most often used by Ness, Mewtwo, and Yoshi. It allows the character in question to attack aerially even faster than with a SHFFL'd aerial.
Instant double jump cancel
Double Jump Cancelling works by allowing the user to perform an air attack while remaining close to the ground (most aerial attacks are performed at a short hop's distance from the ground). However, if the user performs a DJC with frame perfect precision (known as an "Instant Double Jump Cancel" or IDJC), they will actually hit the ground before the hitbox from their air attack comes out. This is true of every attack for every character who can DJC. It should be noted, however, that performing an IDJC is incredibly difficult under normal circumstances, and most often, players will be performing a DJC.
Execution
To perform this technique, the player must do their first jump, then double jump just as they leave the ground, and press A. The player's double jump will be canceled and they will do the attack and immediately fall. The player may fast fall almost immediately, and L-cancel the aerial very quickly. The result will cause the character using this technique to attack extremely quickly.
