The following button combinations can be used on the mission information screens before each level:
- Left + C + Start: adds an extra life
- Up + B + Start: adds an extra Forcer (bomb)
- Right + A + Start: increases the power of your shot
Because pressing A, B, C, and/or Start also exits the mission screen and goes right to the game, only one code can be entered per level (and thus only one of the above stats can be upgraded per level, and by exactly one step each level). Thus, you have to choose what you want to upgrade wisely. (It's like NMK made a cheat code require strategy anyway.)
If done properly, you should hear a chime.
You cannot upgrade a stat past its maximum (8 lives, 8 bombs, 3 shot power levels).