White’s knight attacks h7, and White’s queen is aimed at that square as well: if Black’s queen were gone White could play Qxh7#. In this sense Black’s queen is pinned—to a mating square. What do we do with a pinned queen? We studied this a little while ago. One thing to try is a cross-pin, attacking a queen pinned along a diagonal by adding a rook that attacks it along the file where it sits. Here that would mean White playing Rf4, threatening RxQ. The question about such a move is what White’s rook then would be able to do if Black’s queen moved to play QxQ. The answer is obvious: White plays RxR#, a simple back rank mate. (So after Rf4, the queen becomes pinned to mating squares in two directions.) Since Black can’t afford this result, he must submit to RxQ.