You see that Black’s king is exposed; there is a conspicuous open line—a diagonal—leading to its position. White already has vertical pressure down the g-file. If he could add diagonal pressure by playing his queen to f6, he would be able to mate. The hitch is that Black’s queen guards the mating square, so fix your attention on ways of making that piece uncomfortable. A standard maneuver for the purpose is to stick a rook next to it, as with Re8. Black’s queen could move to d6 and still guard f6 from there, but once White’s rook is on e8 it creates other problems for Black as well: it pins the rook on g8, and thus positions White to mate with Qg7 or RxRg8. Black therefore can’t afford to just move his queen; he has to take White’s rook. Yet he gets mated no matter how he does it. If 1. …QxR, White has Qf6+ and mates next move after Black interposes his rook uselessly on g7. If 1. …RxR, White mates right away with Qg7.

As this position shows, a queen naturally can do the same work usually performed by a bishop in this mate. The beauty of using the queen here is that it also can add other mating threats of its own (i.e., Qg7).