It plays a game of chess against you. The rook moves automatically with it. To go back to the original setup, double click with the right mouse button on the board. To go back one move, click once with the right mouse button. The applet checks legality of the moves, including check.
Promotion is automatically to a queen.
Minor-promotion is not possible. The applet does not detect the end of a game. The computer just makes the first legal move he sees. It is actually more difficult to lose from the computer than to win from it. The computer only plays black.