To do this, press select during a game. To make it come back, press select again.
All this does is make the game harder, so use it at your own discretion.
Everytime you move a piece over, the game counts it. The more you move it, the game thinks you have more trouble with that certan piece, and will dish out more of them. You can exploit this everytime you get a Tetris (Long "I" shape) piece, just move it from side to side as it falls. You will get it more often everytime that you do so.
When you get to the main screen, press the DOWN ARROW key while pressing the START key to begin the game. You will play with levels faster by 10 levels, which means that Level 1 = Level 11, Level 2 = Level 12, and so on.