Difficulty select[edit]
To enter the difficulty "mode" select screen, during the title sequence hold 
and alternately press 
ten times. On the mode select screen press to toggle between "Normal Dog" and "Hard Human" modes. To add additional difficulty modes repeat the above code while within the mode select screen, i.e. hold and alternately press ten times. You can perform this trick twice, the first time adding "Super Mania" mode and the second time adding "God of Game" mode. After selecting your desired mode hold 
and press to return to the title screen.
Interestingly this code can be earned from within the game. It quickly flashes in the lower right corner of the screen once the game is beaten, about one minute after the words "The End" appear.
Sound test[edit]
During the title sequence alternately press 
eight times to activate the sound test. Inside the sound test use and to select a sound, to play a sound, and to stop a sound. To exit the sound test hold and press .