Tetris DX

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

Reverse movement:
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:
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:
Press Start + Select + A + B during game play.

Continue game play:
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:
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.
Information in this section was contributed by RobertBob1258.