Rook and bishop against rook is a draw with good defence, but a hard one to hold. Two methods work. With your king near the edge, keep it and your rook one line in, on your second rank, and shuffle the rook along it. Or, when the kings are at least two lines apart, pin the bishop to its king with your rook, so it can't help with mating threats.
Every move is checked with an endgame tablebase, which knows the exact result of every position with up to seven pieces. The computer defends perfectly, and if a move throws the result away, the trainer stops and shows you a better one.
