You broke a balcony.
To break a balcony you first need to be on a stage with a balcony (Like Dusk after the Rain), then you must use a heavy attack such as <-- + B when your opponent is next to it near the railing. Once you hit them they will come flying down and your partner will be there to greet them to finish whatever combo you may have been doing. If you are playing through Fight Lab then this achievement is impossible to miss as you cannot continue further without doing this achievement.