Turn off the traveling option, then hold X to shoot, but do not release it. Spin or walk behind the defender and dunk the ball.
Various Codes
Enter these codes at the cheat menu:
Dunk from everywhere: DUNKERS
Big Heads: BIGHEAD
Large Basketball: BEACHTIME
Low Gravity: LIGHTY
No lights: DARKNESS
Optional jerseys for the away team: ASECONDJ
Players become skinnier: PRESS
Players become smaller: MINIMINI
Players have bigger feet: BIGFOOT
Players have bigger hands: BIGHAND