The term 20g refers to the block moving 20 units per frame, a.k.a instant drop. The behavior of this mode closely resembles if one were to hold up in TGM2 and later.
A piece cannot move across other minos if the playing field is uneven.
T moved right
One structure that will allow a piece to move either left or right is a pyramid:
With ARS, 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.
Death mode Edit
T.A. Deathis a mode made famous by the "Tetris Japan Finals" video duplicated dozens of times on YouTube, featuring instantly dropping tetrominoes. The difference between death mode and 20g is un-noticeable until later stages in the game in which AREstarts to decrease and lock delay is very low. Arika currently hosts a video showing a GM grade in Death mode here.