Grapple canceling is a technique available to Samus, Link, Toon Link, and Lucas (the characters whose grabs are grapples) in Super Smash Bros. Brawl. The technique allows a midair grapple to be canceled, with no landing lag.
The technique can only be performed while airborne. While any method works as a setup, the most practical method is to short hop. While the character is descending, press grab button just before touching a floor. A character with a midair grapple move will have said move extend quickly with the chance to damage nearby opponents. If done correctly, the move will be canceled upon landing with no lag, and any other move can be performed immediately.
Grapple canceling is most useful with Samus, as her aerial Grapple Beam ("zair") comes out quickly, has a long range, and has decent stun time on it. Samus can also short hop a Homing Missile and start her zair upon completion of the Homing Missile, canceling when it hits the ground. This technique is useful for pressuring, approaching, and stopping approaches.
While other characters with a zair do not have the potency as Samus, they can still use the technique as a spacing and poking tool, keeping opponents at a certain distance as a setup for other techniques and combos.