One unique aspect of Tetris that would apparently defy the earth-bound laws of physics is the existence of a 'lag-time' between when the tetrad meets the ground and when it actually makes contact with the ground (with the thud sound). During this brief moment, one can actually slide tetrads along the floor (or on top of other tetrads, of course.) This technique can often fill covered gaps when they are open from the sides.
If the speed is particularly low, or if you are particularly dexterous, one can slide over two or more spaces at a time (as shown in this advanced example.) As levels increase and lag times narrow, however, such maneuvering becomes all the more difficult.
As one might tell, either the Tetris universe has a very liberal form of gravity, friction does not appear to exist whatsoever, or those saucy tetrads are really greased up... (okay, the ribald humor ends here.)