While you are at the character selection screen, hold L + R + Start. If the code was entered in correctly, the word "Expert" will appear in the lower left corner. No stop or arrow indicators will appear in this mode.

Secret Push Bike

Complete all sixteen Crazy Box levels to unlock the Secret Push Bike for each character which is usable on crazy box, arcade, or original modes. To select the bike in arcade or original mode, highlight a character at the character selection screen, then hold L + R and press Up or Down.