It plays a game of chess against you. The rook moves automatically with it. To go back to the original setup, right click on the board. Also, it enforces that white and black alternately make a move, white starting the game. The applet does not determine stalemate or mate, draw by repetition, or draw by the 50 moves rule.
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.