What is robbing a kong, and when is it allowed?

Robbing a kong means claiming the tile a player adds to an existing melded pung to form an added kong. If that tile completes your winning hand, you can declare mahjong and take it before the kong is completed. This is only allowed when the stolen tile is the exact tile needed to win, and it typically only applies to added kongs, not to concealed kongs. It is one of the rarest and most dramatic plays in the game.