Instructions
Play through the game until the fourth quarter. During the fourth quarter of the game, the chances of breaking your opponent's backboard increase significantly.
Continue playing until you are inside the last two minutes of the game. Your chances of breaking the backboard increase again when you are inside the game's final two minutes.
Run toward the opponent's hoop while holding the "Z" button to use your turbo run.
Flick the Wii Remote up and then down while you are approaching the hoop to perform a slam dunk. You can only break the backboard while doing a slam dunk.
Repeat Steps 3 and 4 until you break your opponent's backboard.