If you have a winning hand of groups and runs using at least one joker, you do not have to expose it immediately. If you wish, you can continue playing in the hope of forming a winning hand plus a joker. If you are able to end the game by discarding a joker and exposing your remaining 14 tiles as a winning hand, your win is worth twice as much as an ordinary win. Note that by continuing to play instead of exposing your ordinary win, you run the risk that another player may complete a winning hand and expose it before you can achieve your double win, in which case you gain nothing for your concealed winning hand.