Rules in brief
Ivory moves counter-clockwise toward the bottom-right home board; Ebony moves the opposite way. Each die is a separate move, and doubles play four times. You can't land on a point held by two or more enemy checkers. Landing on a single enemy checker sends it to the bar, and it must re-enter before anything else moves. Once all fifteen of your checkers are home, bear them off. You must use both dice when you can; if only one can be played, it must be the larger.
Before rolling you may offer the cube. Your opponent either passes and loses the current stake, or takes and plays on at double the stake, owning the cube. A gammon (the loser bore off none) scores double; a backgammon (the loser still has a checker on the bar or in the winner's home) scores triple. Under the Jacoby rule, gammons count only after the cube has been turned. In a match, no doubling is allowed in the one game right after a player first reaches one point from victory (the Crawford rule).
About the engine
The computer evaluates positions with a neural network first trained by playing against itself, the method behind TD-Gammon, then retrained on 300,000 positions evaluated by GNU Backgammon at 2-ply, with inputs for strategic features such as primes, escapes, home-board points and shots. For each position it estimates the chances of winning, winning a gammon or backgammon, and losing one. Expert strength also looks one roll ahead for its best candidates. Cube decisions use these estimates with Janowski's live-cube model for money play and the Kazaross-XG2 match equity table for matches.
Grades compare your play with the engine's best play in cubeless equity: under 0.04 is good, 0.04 to 0.08 doubtful, 0.08 to 0.16 an error, and 0.16 or more a blunder. PR (performance rating) is 500 times your average equity loss per move, the same scale eXtreme Gammon uses, so lower is better: under 2.5 is world class, under 5 expert. Both are measured against this engine, which is strong but not as strong as the top commercial programs.
Huckleberry