Black has the kernel of a discovered attack on the f-file. The potential target for his rook is White’s rook on f1—which is protected, however, by the enemy king. Black’s queen is trained on the rook already, so the solution is an exchange: Black plays QxR+, requiring the reply KxQ. Now the king is on the f-file, so when Black moves his bishop he has a discovered check. But instead of looking for White pieces to attack with the bishop, think first about ways to give check with it—i.e., double check—and go for mate. Bd3++ forces the king to move. This time it can go two ways: back to g1 or on to e1. Either way Black executes the now-familiar mate pattern with Rf1#.