Jack Bros.

• compatible with the North American version only.
• does not apply to the Japan ROM but you can use the secret language-swap button code built into the original game (!). Hold , and then press on the title screen; this will switch languages and save the new setting to SRAM
• adds ability to enable/disable the debug mode. Hold while booting the game; this setting is also saved to SRAM. The title screen shows “DEBUG” in the bottom right when enabled
• enables the Japan-only sound test and fixes the Japanese text to display correctly
• separately saves progress for all “three” characters from the Select Player screen and/or by entering passwords, i.e. passwords will replace your save games
• saves the difficulty setting (normal/”for super players”), with separate save slots for normal and super difficulties
• adds secret character to the Select Player screen. Hold + while cycling through characters
• erase individual character saves by pressing four times in a row while they are highlighted on the Select Player screen. There are warning sounds

Sources here.