"No Spin Tetris" started out with the idea of whether or not it is possible to make a fun version of Tetris where, instead of rotating the pieces, the player has to change them in another way.  To do so, the pieces would have to morph instead via shifting, growing, or shrinking.

After lots of trial and error, 7 pieces were settled upon that do just that (while complimenting each other gameplay wise as well).  It might seem quite difficult at first, but that is only because you end up having to think about the gameplay in a completely different way than other variations of Tetris.