L-canceling
L-canceling, or Z canceling in Super Smash Bros. (officially called a smooth landing), is a technique in both Super Smash Bros. and Super Smash Bros. Melee for reducing the length of recovery animations after landing on the ground while performing an aerial attack. By pressing Z, L, or R (hence the names "Z" and "L" canceling) slightly before landing on the ground, players can reduce their recovery time, a significant decrease that greatly increases a character's maximum rate of attack. L-canceling is no longer available in Super Smash Bros. Brawl, though auto-cancel is in.
Technical details
To L-cancel, the player should press L, R, or Z (only Z and R work in SSB) within 20 frames of landing on the ground while the character is still performing an aerial attack. If performed correctly, the character's recovery animation will be played at twice its normal speed in SSBM, effectively cutting recovery time in half. If the number of frames of the lag divided by two ends with a decimal (e.g. 15/2 = 7.5), it will be rounded downwards. In SSB, however, the recovery animation is erased, allowing one to move immediately after any aerial attack. L-canceling can only be used with aerial attacks involving the A button; it is impossible to L-cancel a special move. It is also impossible to L-cancel Mr. Game & Watch's neutral aerial and up aerial attack.
Auto-cancel
Auto-canceling occurs when the character lands at the earliest or last frames of an aerial attack. This causes the character to land with normal landing lag (usually 4 frames) without any additional lag from using the attack. When an attack is auto-canceled, L-canceling has no effect.
