Tetris DX - Platform: Gameboy Color

Expert mode
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Press Start to pause game play, then press Select. The preview 
window will not show the next piece when game play resumes. 

Reverse movement
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Hold Left or Right while any piece except a straight bar or block 
is descending until it is against the wall. Then, keep holding Left 
or Right and rapidly tap A. 

Alternate screens
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Allow the game to remain at the main screen until demonstration mode 
begins. Press Select to display a screen featuring swimming fish. 
Press A to return to the next part of demonstration mode, then press 
A again to display a screen with confetti and the Nintendo logo. 
Press B to return to the next part of demonstration mode, then press 
B. Eventually a screen featuring a blackboard will appear. 

In-game reset
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Press Start + Select + A + B during game play. 

Continue game play
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Press Start to pause game play in single player mode, then turn off 
the Game Boy. A continue screen will appear when the Game Boy is 
turned on again. 

Screensaver
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Remain idle at the main menu for about 20 seconds. A demo game will 
begin. Press A and remain idle for another 20 seconds. Fish will appear. 
If you repeat these steps, flying triangles will appear.