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One interesting feature in more advanced tetris games is the ability to set one block aside for later use. When the user wants to use it, the block switches with the currently-falling block, which is in turn stored and you have to let the block set, you can't switch it again until a new block appears.

This is probably something that users might want to turn on or off, but that is not really necessary because if someone does not want to use it they can just not push the keyboard button that triggers it.