Black has no bishop checks (Bc5 obviously would be unsafe, but it's irrelevant because the bishop is pinned). Think more broadly about threats he might make with the piece. White’s bishop, knight, rook, and queen all are on light squares, and so is Black’s e4 bishop. The queen and rook are on a common diagonal and so can be attacked with Bc6. When a bishop forks the enemy queen it requires protection, as we know; it has the needed cover here from its own queen at c1. Black thus wins the exchange: 1. …Bc6; 2. RxBf8+, RxR. Or perhaps he wins a queen for a rook and a bishop, the result if play goes as follows: 1. …Bc6; 2. QxB, QxQ; 3. RxR, Kg7 (protecting the bishop). (Or instead 2. RxR, BxQ; 3. RxB+—same basic result.)