The beginnings here are familiar: a bishop on the same diagonal as its king, not itself able to be pinned but on a square that is vulnerable to a pin if a better target is put there; White has a bishop ready to deploy to b2. So White forces a substitution: RxB. Notice that Black can’t reply d6xR because the d6 pawn is pinned to his queen—a case of a relative pin, and a pattern we will study in detail later. Black can recapture only with QxR. Now White pins the queen with Bb2. Since the queen can strike back against the bishop (or any other piece that pins it), the piece imposing the pin requires protection; here the bishop gets backup from the rook on b1.