Notice that Black’s bishop on g7 is pinned, and that White has both his bishop and queen aimed at g7, even if the bishop’s path isn’t clear yet; in this sense Black’s knight on f6 is pinned as well. How to exploit the situation? Play through what you would like to be able to do. White would like to be able to play BxNf6, taking advantage of the pin on the g7 bishop and following up with QxB#. He can’t, because Black’s queen guards f6. So he turns his attention to the queen. He can attack it with Rxd7, which incidentally forks Black’s loose bishop on b7 as well. Black is confronted with a familiar choice. He has nowhere to put the queen where it will be safe and still protect f6; if he tries to save his queen by playing QxR, he gets mated. He is better off letting go of the queen and looking for another way to fend off the mate threat, such as e6-e5. Now White plays RxQ.