This is Beat's ability, which he obtains once he gets the (S) Air Cancel sticker.

When on ground, an air cancel will make Beat jump and switch to an aerial combo in the opposite direction. Like any other partner, attempting on air will only stop the attack, with no further effect.

In-game text: Beat can jump while in the middle of performing a combo. This is called an air cancel. Just press the button for the direction opposite the way Beat is attacking. Air cancel mid-combo to avoid danger, then switch to an aerial combo and keep delivering the hurt!