From wiki
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, flatten the field, or allow a player to tetris.
Skimming is especially effective in games that use combo scoring. In other games, skimming forfeits "back-to-back" bonuses and is penalized with more line clear delay per line.
[edit] Minimize I tetromino dependence
When a player is starved of I tetrominoes, he has to depend on other tetrominoes to keep himself from topping out or to keep his field stable.
A T can be used to skim, either directly or by creating an overhang to a T-Spin Double.
[edit] Funnel
A field may not be set up for skimming without covering the open column. Stack around the open column to create a funnel for skimming efficiently.
A funnel may be created by filling up the necessary cells to create the indent.
If there is already an indent, fill up the cells to create the wall.
[edit] Inverted Funnel
Funnels don't have to be limited to long and empty single columns.
