It plays a game of chess against you. The rook moves automatically with it. To go back to the original setup, click with the right mouse button on the board. If you have a one-button mouse, click and press the alt or control button at the same time. The applet checks legality of the moves, including check.
Promotion is automatically to a queen.
Minor-promotion is not possible. The computer plays very bad. It does a 2-ply deep minimax search, but with a rather poor evaluation function. The computer only plays black. The current version uses the event model of the JDK 1.1 or 1.2.