Instructions
Bring the basketball across the half-court line and into your offensive zone of the court. You can't do this maneuver from your defensive half of the floor.
Hold down the "L" and "R" buttons at the same time.
Press "X" while still holding the buttons down. If your intended target is not well-guarded by a defender, he will dunk it.
Turn to your opponent and talk all the trash you want. If you talk too long, though, he'll be doing the same thing back to you.