Connect two controllers to the NES Deck, power on the NES and on controller 2, press and hold SELECT, START and A during the copyright screen and release the 3 buttons just as the screen dissapears.
Then you'll be taken to a Sound Test menu. If you hold the buttons for too long you'll just end up on the title screen.
In the sound test menu play one of the tracks called "sample" which are number 25 - 29. Once that's done select "off" which is number 0, then press SELECT 3 times on controller 2 and you'll see a secret
message. Or you could just check out the screenshots below :-)
In the sound test menu select "Off", that's number 0, and press A 10 times. When the code is correctly entered a tune will play. Then press START to return to the game and on the team selection screen,
press UP to select an invisible team, called "Beam Team". It doesn't matter which controller you use.
In the sound test menu select "Theme", number 10, press A 10 times. The music will stop if it's done correctly. Then press START to return to the game and now you can pick an extra delivery method
with a highly screwed ball. On the delivery selection screen hold DOWN to choose "Short" delivery and press A or B to select fast or medium bowler. Instead of a short delivery you will be able to throw