A Jump-canceled grab (abbreviated "JC'd grab") can be performed by jump-canceling a run or dash with a grab, by attempting to grab while still in the jumping animation. This is usually done by pressing X, Y, or up on the control stick, then pressing Z immediately afterwards. The character performs the standing grab animation while sliding forward slightly (depending on traction). For most characters, JC'd grabs have less startup and/or cooldown lag than standard dash-grabs.
Jump-Canceled grabs usually have slightly larger hitboxes than standard (though still smaller than actual dash-) grabs, and are primarily used in chain-grabs with throws of forward trajectory (such as Captain Falcon's down throw) or involving enemy DI (such as Mr. Game & Watch's up throw chain grab.)
JC'd grabbing does not work in the original SSB.