Attack cancelling is a technique in Super Smash Bros. Ultimate done by inputting a ground attack (i.e. jab, tilt, dash attack, smash attack) and immediately jumping within the initial frames; this allows the player to buffer an aerial attack out of a short hop frame-perfectly.
If the player has their C-stick set to "Attack", flicks it in the opposite direction they are facing, and jumps immediately, they will buffer an aerial in that direction without having to first turn around. If the player does this while dashing, they will perform a back air towards the direction they were running in. This means they can easily reverse aerial rush out of an initial dash, facilitating extended combos and surprise attacks; a retreating forward air is also possible this way. Characters such as Captain Falcon benefit heavily from this, as it buffs their already strong combo games by letting them use more aerials in more ways while conserving momentum from the dash.