DTET was a fan game created in Japan with the goal to create the author's own vision of Tetris, different from both the guidelines or TGM. It implements many rule elements that are not seen elsewhere, such as symmetric wall kick (elaborate?), double rotation, a huge number of previews, and on-demand ARE canceling (No line clear delay in ADVANCE or FINAL level).
The game's control system consists of 2 rotate buttons, unlike many other Japanese games which feature a duplicate button for one of the directions (usually anti-clockwise). This potential drawback is overcome as follows: When a piece is rotated, and the other rotate button is presssed while the first button is still down, the piece rotates an extra turn in the direction of the first button, for an overall 180 degree rotation, otherwise known as a "double rotation". One can also produce an instant double rotation (treated as counterclockwise for wallkick testing purposes) by hitting both buttons simultaneously.
DTET features score attack, time attack and battle modes, as well as two special ("FINAL") modes.
Score attack is your basic marathon mode, where getting a certain number of lines (usually 8, sometimes more in HARD mode) nets you a level, and you have to survive as long as possible and get as many points as you can. DTET gives you five lives in this mode: if you top out, you can start over from the bottom as long as you have lives remaining.
Time attack tests how fast you can clear 100 lines at the maximum level for each mode. You get three lives, as well. Clearing all the required lines without losing a life rewards you with a Perfect!
Battle modes pit you against other players (over LAN) or against computers. The Official Battle menu lets you select an AI strength and game difficulty (mode and level).
FINAL modes are for experts only. Joker mode starts a game at 20G (level 50) where each line clear increments the level counter. Making a tetris adds one to your stock. When you reach level 200, only tetrises count toward your level and any other line clear takes one out of your stock. If you have no stock left by level 300, the game ends, but if not you can continue past level 300 as long as stock remains. You only have one life in this mode. Furthest mode is a hard 20G challenge where you have ten lives to clear 300 lines. All of the delays in this mode are incredibly short. Both of these modes blend the pieces in with the background, only showing an outline of the stack to the player.
There are three different modes which can be applied to the score attack, time attack, and battle mode. In each, there is a drop in gravity at level 20 as well as level 40 (except in NORMAL, where the game never reaches 40). Levels 50 and up are 20G.
NORMAL mode caps off at level 30. There is no hard drop in this mode.
HARD mode caps off at level 50. In score attack, the number of lines per level steadily grows over the course of the game. Level 50 requires 500 lines.
ADVANCE mode caps off at level 200, but also includes a timer at the bottom of the playfield. Line clears will restore the timer, but if it hits 0, your game is over no matter how many lives you have left. In score attack, each level past 50 is earned with one line clear.
Official DTET Website (note: game is no longer available for download there)