L-canceling (an abbreviation of lag canceling or L button canceling, also called smooth landing on the official Super Smash Bros. website) is a technique in Super Smash Bros. and Super Smash Bros. Melee that allows characters to act faster than usual when landing in the middle of an aerial attack. L-canceling is done by pressing a shield button 11 frames or fewer before landing during an aerial attack in Smash 64 (the grab button also works because of its unique properties), or by pressing the shield button 1 to 7 frames before landing in Melee.
Only regular aerial attacks can be L-canceled; special moves that have landing lag cannot. If an attack is auto-canceled then L-cancelling has no effect. All aerials can be canceled except Mr. Game & Watch's n-air, b-air and u-air.
In Super Smash Bros.
Better known as Z-canceling as the Z button is the shield button, L-canceling in Smash 64 is performed by pressing the Z button 11 or fewer frames before landing. Doing so causes the character's regular landing animation to play instead of the air attack's landing animation, reducing their lag to 4 frames. Compared to lag intervals in the 20-30 range, this can allow incredible attack speed when combined with techniques such as double jump canceling, resulting in shield break combos.
In Super Smash Bros. Melee
L-canceling in Melee is performed by pressing L, R or Z up to 7 frames before landing. Doing so causes the aerial attack's landing animation to play at double speed, cutting the lag time in half (rounded down). While not as potent as the previous game, the decrease in lag is still significant enough to make L-canceled aerial attacks better than comparable ground attacks. This is especially true for characters with heavier aerial landing lag, most notably Bowser and Ganondorf.
In later games
L-canceling is not present in Brawl. It was removed because, like wavedashing, Masahiro Sakurai felt it made the gap between beginner players and advanced players too large. The lack of L-canceling acts as an indirect nerf to characters that have aerial attacks with heavier landing lag and/or low jumps (most notably Ganondorf).
L-canceling continues to be absent from Super Smash Bros. 4. However, equipment may have the Smooth Lander effect, which automatically provides a similar result. Depending on the number of times the effect is stacked, landing lag from aerials can be cut to 0.7x, 0.6x, or even in half as in Melee. Additionally, the landing lag of most aerial attacks from Brawl have been universally reduced to properly compensate for the loss of the technique, notably those of Ganondorf.