Start a game and quit by pressing START+SELECT. At the main title screen press UP, UP, UP, DOWN, DOWN, DOWN, RIGHT, RIGHT, TRIANGLE. You will hear a chime.

To enter a special debug mode, wait for the Core Design copyright message to appear, then quickly press Up, Down, Left, Right, Down, Down, Right, Right, Triangle.

You will be taken to a test menu where you can choose your starting level, max your firepower, view all the videos, see the credits, and improve the game's appearance.