Pop the balloon with the next book move until the opening stands on the board.
An opening stands at the top, for example Italian Game. Balloons with moves rise from below. Pop the balloon with the next book move, for White and for Black, until the opening stands on the board.
The other balloons are real branches into other openings or simply possible moves. A wrong balloon or a book move that escapes costs a life. The earlier you hit, the more points you get.
If you play one side, the other moves by itself.