At the start and options screen press B, A, B, Y, down, right, A, L button, down. If done correctly you should hear, "Yip!". To use it anytime during a game hold the L button, and press up to advance a level, or down to go back a level. You can fight the bosses and the dragonfly race this way also. Or you can hold the L and R buttons and press start and it will bring you to a menu which will let you select a level you have played before. You can only access this a few times, the counter on the far right tells how many times more you still have this option.
After you complete the first level, kill yourself. When the press start screen appears press and hold right, then press start. At the next screen hold right and press start.