Skimming

Skimming is a clearing method by which a player can use to make a stack even. Skimming is done by placing a tetromino either at the edge or at the surface of a stack and cleaing a line as a result. The technique is favorable in multiplayer tetris because it can save a player from topping out or allow a player to tetris.

Downward
When a player is starved of I tetrominoes and garbage is coming at a fast rate, he has to depend on other tetrominoes to keep himself from topping out.

If a player is close to topping out, create a funnel for safety.

Easy to grasp? How about these?

Upward
When unable to tetris because of a funnel, fill it up.