Press A eight times, B once, A seven times, B once, A six times, B once, A five times, then B three times at the title screen - a sound confirms correct code entry. You can pause the game during gameplay to toggle the mode on and off. Additionally, you always have 100 health and your status is periodically healed. You can also walk through walls and doors without having the keys required for them.
At the title screen, press A, A, B, A, B.
Press A, A, A, A, B, B, B, A at the title screen.
At The Start Screen: press A(x8) B(x8). This will give you the password screen. Here are the passowrds for each part of the game which can be used on the password screen:
Hold Up, A, B, Start, and Select simultaneously and then reset the console. Keep holding the buttons and eventually you'll see a hidden version number message.