Crown Landing is an advanced technique in Super Smash Bros. Ultimate discovered by Plague von Karma, that allows King K. Rool to significantly reduce—or if performed frame-perfectly, completely erase—his landing lag. This is executed in two ways, referred to as the "No Crown" and "Crown Pickup" variants by the playerbase.
How to perform
To execute the No Crown variant, the player must input King K. Rool's side special move Crownerang while the crown is not on K. Rool's head between 35 and 45 frames before landing. This will have K. Rool land a few frames before the "No Crown" animation ends (which lasts 36 frames), converting into his landing lag in an unnatural way. If done perfectly, this will leave K. Rool with either only one single frame of landing lag (rather than six from the hard landing), or snap him into his idle animation. Thus, it is possible to land and then immediately perform another action.
To execute the "Crown Pickup" variant, the player must collect K. Rool's crown 35 frames before landing. This will almost always leave the player with either very little (one frame) or absolutely no (zero frames) landing lag, being much more generous—albeit situational—than the "No Crown" variant. The biggest benefit of this is that it allows K. Rool to use Crownerang immediately after landing, enabling a lot of horizontal pressure reminiscent of another Crownerang-based technique called Crown Squatting. In particular, this enables a lot of pressure around Battlefield's platforms, as they provide a very safe way of executing this tech.
King K. Rool tech: Crown Landing execution
Crown Landing in practice