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.