If you win the World Champion title you will get at secret boxer that you can use in Exhibition mode. If you get all champion titles you will get all secret boxers.
During a bout in Road to World title or tournament mode; Hold L1 + L2 + R1 + R2 and press X. Then press X again to display a loading screen instead of the game trivia. Now press X again to win the current bout.
Thanks to Revolution reader Henrik Pedersen!