20G

The term 20G refers to the gravity behavior where the active tetromino moves downward 20 cells per frame. In modes that feature this behavior, the active tetromino spawns directly above and is in contact with the stack. Lock delay is a necessary component for 20G to be playable. The TGM Series is particularly well known for its emphasis on 20G gameplay. xqit spycam

20G vs. 0G
Under 20G behavior, a tetromino cannot traverse freely over a column if there are blocks obstructing its path, unless a wall kick or floor kick is used.

In contrast, the O tetromino under 0G can freely move left because there are no obstructions at the top of the playfield.

Pyramid Structure
To combat the limited mobility of tetrominoes under 20G, a player should stack a little higher in the center relative to the sides. Since the active tetromino spawns in the center, the player would have an easier time traversing it to the left or right side of the playfield. Different rotation systems have varying spawn orientations that may aid or hinder mobility under 20G.

Under ARS, the rotation system used in the TGM series, every piece has an orientation achievable during entry delay that will allow it to land on the tip of the pyramid -- column 5. From this point, the piece can be moved either left or right as necessary. Having column 5 higher or on equal level with surrounding columns helps maximize the number of moves that can be performed with a given tetromino. Still, the player should make an effort to avoid overstacking in the center.