Relevant parts of the Wikipedia article linked to by parent:
> Tetris 99 is a multiplayer puzzle game in which 99 players play against each other at the same time, with the aim to be the last player remaining. [...] As with normal Tetris rules, players have the option to store a tetromino piece to swap out at any time. By clearing multiple lines or performing continuous line clears in a row, players can send "garbage" to other players, which will appear on their board unless they can quickly clear lines in response. More garbage can be sent by completing combination moves in succession of making a "tetris" (matching 4 lines at once) or performing a "T-spin" (squeezing the T-shaped tetromino into a position it would otherwise not fall into by rapidly rotating it).
> Tetris 99 is a multiplayer puzzle game in which 99 players play against each other at the same time, with the aim to be the last player remaining. [...] As with normal Tetris rules, players have the option to store a tetromino piece to swap out at any time. By clearing multiple lines or performing continuous line clears in a row, players can send "garbage" to other players, which will appear on their board unless they can quickly clear lines in response. More garbage can be sent by completing combination moves in succession of making a "tetris" (matching 4 lines at once) or performing a "T-spin" (squeezing the T-shaped tetromino into a position it would otherwise not fall into by rapidly rotating it).