Tetris DX
Expert mode
Pause the game and press Select. Then the preview window will not show the next piece when game the resumes.
Reverse movement
While any piece except a straight bar or block is descending, hold Left or Right until it is against the wall. Then, keep holding Left or Right and rapidly tap A.
Alternate screens
Allow the game to remain at the main screen until demonstration mode begins. Then, 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.
Continue game play
Pause the game in single player mode, then turn off the Game Boy. A continue screen will appear when the Game Boy is turned on again.