Tetris DS is not the only game to acknowledge T-Spins, but other games have different criteria for what a T-Spin is. This is what Tetris DS uses to identify a T-Spin:
The last movement of a piece must have been a rotation, ie. not a movement either downwards or to the side.
Three of the four corner squares must be occupied by wither other pieces or the wall of the playing field. To try and explain this:
Imagine the four rotations in a 3x3 box like this:
For the rotation to be considered a T-Spin by the game, three of the four corners must be occupied. Here are some examples:
All of these would be classed as T-Spins by the game, and a box would flash 'T-Spin' on the right of the playing field, confirming so.
The fact that the system works like this allows for some moves which shouldn't be T-spins:
As you can see, the piece could have been dropped in place, but this method registers it as a t-spin, and awards extra points/ garbage lines because of it. There are also some rotations where you might expect it to be a t-spin, but it isn't as not enough corner squares are occupied:
Using T-Spins On Single Player[]
The only way to get huge scores on marathon and line clear is to exploit the huge bonus points which t-spins give per line over Tetrises. The main two setups which you can use to get points are the double and triple setups:
It is helpful to know these setups and to be able to recognise them. This allows you to make them with improvisation (rather than by following certain setups), converting a possibly awful field into a very beneficial one.
T-Spin triples are pretty useless on their own. While making them it can cause you to stack high around the setup, and once the T-Spin is done, you are left with an awkward and unbalanced playing field. To counter this triples are usually setup back-to-back, to get the greatest return for a messed up playing field.
The main benefit to this setup is that it can allow the use of T-spin doubles indefinitely. In practice however, it is usually not quite that simple. The main shape requires a continuous back-to-back t-spin triple setup (two blocks, gap, two blocks, gap etc.) which is three blocks away from the wall of the playing field. This shape needs to be built without using S or T blocks, as they are needed to complete the doubles. Here is a basic portrayal of how it works:
