Figure[White to move]

Black moved his king out toward the middle of the board, which is hazardous in the middlegame for many reasons (it typically becomes a good idea in the endgame, but he wasn't quite at that stage yet). One reason is that it is easier to pin things to the king once it is surrounded by accessible lines. Here White senses opportunity but has no immediate way to impose a pin, so he considers his checks and their consequences. Again there are two pieces to consider: the rook and the queen. Rc6 loses the rook, and Rf5 loses the rook (and anyway it would allow the king to escape onto the sixth rank; notice that b4 is off limits). So how about the queen’s checks? Qg5 is of great interest because the sixth rank remains sealed off by the rook. Black’s king thus has no flight squares, so Black will have to interpose something between the king and White’s queen. If he plays Re5, the rook is lost to QxR+ and White mates soon. If Black instead plays Qd5, Black’s queen will have walked into a pin.

How to exploit it? Again, look for other pieces (besides the White queen imposing the pin) that can be used to take Black’s queen now that it's stuck. White’s only plausible piece for the purpose is the rook, so try Rf5. This way the rook takes over the pin (its services no longer are needed on the sixth rank; its purpose there was to force Black to pin his own queen on the previous move). And notice, incidentally, that Rf5 also unmasks an attack by White’s queen on Black’s rook. The best Black can do is play his rook to d7 so that it can recapture when his queen is taken. Now White has RxQ; Black replies RxR, and White has won a queen for a rook.