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 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.