The pin White’s queen exerts against Black’s queen on the seventh rank is such an arresting sight that it would be easy to be consumed with thoughts of exploiting it somehow; alas, Rd3 doesn’t prevent Black from playing QxQ. What would? A check—so examine any that White can give. He has two with his queen that are unhelpful, but one with his rook—Rh7—that is helpful indeed, as it forces the king to move and so exposes the queen to capture with RxQ. The pin turns out to have been a sideshow. Notice that Black could also reply to 1. Rh7+ by interposing his own rook at f7. But that gives White RxR+, and Black's king cannot recapture because the bishop at c4 guards the f7 square; instead Black's king must move, again leaving his queen vulnerable.