At the title screen with the Slime World logo that falls with a crash, do the following:

1. Press and hold Option 2
2. Press and hold Option 1 while holding Option 2 down
3. Move the joypad to the upper right, while holding down both Options
   

After that, release your fingers, and if desired, turn the music back on. There is no indicator of your success, until you start playing. Things behave very strangely in multiplayer mode tho, so be warned!