White has the kernel of a discovered attack on the d-file, where the bishop masks the rook. What prevents this from being an effective discovered attack, or indeed a discovered check? First, White’s own queen blocks the rook’s path up the file; second, there is not yet a Black target on the file. The solution to these problems becomes clear enough when they are viewed together: use the queen to bring the king into range. Thus Qd8+ forces KxQ. Since White will have sacrificed his queen, he will be interested in mate; can he get it? Bg5++ gives check with both bishop and rook; Black must move his king. He can take it two places: back to e8 or to c7. If he plays Ke8, White follows with Rd8#, using our current pattern. If Black plays Kc7, look for White’s next check: he plays Bd8#, using something like the reverse of the current pattern. The bishop attacks the king and cuts off the flight squares on its diagonal; the rook protects the bishop and cuts off the Black king’s remaining flight squares. (White could have made a different second move, of course: Ba5+, giving the double check from the other side of the board. But it doesn't work; Black's king soon can escape to e6.)