Figure[White to move]

Here again is the position you want to achieve. White sees a possible pin of Black's queen. He can impose it with his bishop or his queen. Obviously he will use his bishop so that the pin is profitable, but it will need protection to prevent Black from just replying QxB. Fortunately the pinning square—c4—already is guarded once by White’s queen, so he can play Bc4 without concern. Black’s queen is lost; it can’t play QxQ, and instead has to settle for taking White’s bishop and then being taken itself.

Of course White also can make slightly lesser gains here by playing QxQ; then after the recapture RxQ he has Bc4, pinning the rook and winning it a move later. It’s a classic substitution of a harder-to-pin piece with one that is easier to pin—but it’s not necessary.