Begin a race. As the lights count down, wait until you see the yellow monkey. When he appears, press and hold X to start the race with a turbo boost.

To unlock every race and racer, go to the title screen and press CIRCLE, TRIANGLE, CIRCLE, SQUARE, X, SQUARE, X, TRIANGLE, TRIANGLE, X, SQUARE, CIRCLE. Then go to the Challenge screen and press X to select a challenge. You'll have access to all riders and every race. To infinity and beyond!