It's a game where irregularly shaped Blocks fall from the top of the screen and by rotating them you find a place to put these ‘tetrominos’ within a limited play space. Filling a line with blocks makes it disappear but the challenge comes from not all the blocks tessellating, as this leaves gaps which you can't fill owing to others blocks obstructing access to them. Leave too many gaps and the blocks have nowhere to fall to and when the screen is full the game is over. It's a game about finding order in a world of chaos. It's about lateral thinking and space manipulation. It's the "tidying game".
The first player for example can advance a level and start playing 'Dr. Mario' while the second player is still in a 'Tetris' stage. Seeing a similar looking yet different game in your peripheral vision is disorientating. Also since they two players can be playing different games at any given moment what they do has no bearing on their opponent. Traditionally much of the multiplayer enjoyment in these sorts of puzzle games comes from your success punishing your opponent. For example, clearing multiple rows of bricks in your Tetris grid sends incomplete rows onto your opponent’s grid. They then have to clear this "garbage" while still dealing with their falling bricks. This makes the game feel like a competition, and without your success punishing your opponent 'Mixed Match' mode ends up being a simple race. If you're playing alone however it's a race with no active opposition as there is no opportunity to play against the computer in 'Mixed Match'.