Basically, the idea is that for a Tetris stage, you'd play on the falling blocks and try to avoid getting crushed. However, one player could opt not to play Smash, and instead actually PLAY the Tetris level as its own game! The Tetris player would be the one putting the blocks down, and would attempting to crush the other players, or trip them up, or generally just interfere with the match.
If you crush a player, you get credit for the KO, so you can still participate and even win the match.
Of course, something things would need to be changed for this to work, the first of which being to give all characters a wall-jump so that they can't be stuck in a long tube. But it would still be awesome.