Swap and Fusion combine both Puyo Puyo and Tetris in different ways. In Swap, two boards–one for Puyo Puyo and one for Tetris–are managed alternatively after a set time. This is the most balanced mode that combines both game types and works very well. It adds an extra layer of strategy, forcing players to time combos so that garbage is repeatedly sent to the same board to maximize nuisance.
In Fusion mode, puyos can change colors, pieces switch between tetriminos and puyos as they drop, and frustratingly, once a tetrimino touches anything on the board, it can’t be moved. It becomes increasingly more difficult the higher the board is stacked, preventing you from trying to set up large combos with puyos. Also because tetriminos can’t be moved once they touch something, they can’t be slid into an open slot beneath a block as in regular Tetris mode, either. So good luck if your first tetrimino is an S or a Z. The saving grace is that tetriminos clear the garbage it falls on, and it does add a whole new level of strategy to both Puyo Puyo and Tetris, as a falling tetrimino will reposition any puyos it passes. There’s a steep, steep learning curve to Fusion, as it doesn’t always follow the rules of the individual games.