The pin already is in place, this time of White’s rook by Black’s bishop. The rook is protected once as well as attacked once, but since the bishop that exerts the pin is worth less than the targeted rook it is possible for Black to simply play BxR, winning the exchange after Black recaptures. Possible, but not optimal; much better is to try to take the pinned piece cleanly by coming after it a second time. Thus if Black plays Rf2 White’s rook is attacked twice and protected only once, and White has no way to add more protection. The White rook gets taken on the next move, and Black can't afford to recapture. Black wins a piece rather than just the exchange.

The point of the position is that even where the pinning piece profitably can take the pinned piece, piling on superior force against the target sometimes is the better course, leading to a decisive gain rather than just to a favorable trade. Incidentally, note that Black had two ways to add pressure with his rook: Rf2, but also Rb3. What’s wrong with Rb3? It allows White to add protection to his rook with Kg2—a move not possible if Black’s rook is on f2. Moral: when you have more than one way to add pressure to a piece, think about how your decision will affect the choices available to your opponent.