From Hard Drop - Tetris 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.
 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.
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.
 Inverted Funnel
Funnels don't have to be limited to long and empty single columns.
 Line Clear
A line clear can be used to create a funnel.
In this example, using an L to clear the far right column will not result in a funnel. If an I was to be used instead, a JZ funnel will be created.
Tetrominos can create different funnels depending on their orientation when used to clear a line.
Different tetrominoes can create the same funnel depending on their orientation when used to clear a line.
Funnels can be formed by rotating tetrominoes under overhangs.
This only works in SRS.
This only works in ARS.
This works only in SRS with the exception of TGM3's Classic rotation system (an ARS variant), where the T is given a floor kick.
Twists may be used to create funnels.
These only work in SRS.
 Alternative Funnel
Funnels can be set up without utilizing the open column.
 Suspended Funnel
Funnels can be extended over a column.
Funnels can be mirrored with various line clears that raise the wall on the opposite side.
The concept can be taken further. In this example, the S tetromino can convert the LS funnel into a JZ funnel. The process may be repeated until the wall on either side of the funnel becomes too short to skim any further.
Inverted funnels can be mirrored efficiently with a J/L or T tetromino.
When unable to tetris because of a funnel, fill it up. The main idea is to fill up more rows than the amount you clear. Technically, the I tetromino would be the best choice since it clears no rows, but it requires the player to have another I tetromino on hand. Therefore, the next best choices are the L, the J, and the T because they clear only one row for every 2 they complete. The S/Z tetromino clears two rows for every two rows it completes, which cancels out any effects.
Step pyramid and castle structures may become an obstacle when stacking for a tetris. Only specific tetrominos can fit, which limits a player's placement options. Minor line clears can be performed to flatten the structures.
When skimming consecutively, the order in which tetrominoes are used can make a difference. Tetromino properties have to be considered.
In this situation, clearing with an S first eliminates the funnel, disallowing any further skimming without covering the open column.
Instead, if an L was to be used first, the funnel would still exist.