Tetris: The Grand Master

"...after reaching level 500, the falling pieces cease to have an airborne phase, and instead will instantaneously appear at the bottom. This state is referred to by developers of the game as 20G. This change in behavior modifies the gameplay mechanics significantly, to a much more strategic game. When previously players would align the horizontal position while the piece is still airborne, and then drop the pieces, this becomes impossible since the pieces are already at their lowest possible vertical positions when they appear. Instead, players are required to quickly trundle the pieces along the terrain to get them to their intended locations, before they lock down. Similar conditions can be observed in some modern iterations of Tetris, such as Tetris DS." - Read more...

More info here.