This time White's bishop on b5 is the piece with a limited ability to retreat. When you see a piece trapped in this way, think about threats against it. For Black that means considering Rd5 here. The bishop then gets taken if it moves to e2, d3, c6, d7, e8, or a6. Its only safe move—and it's only temporarily safe—is Bc4. (If White tries Be8, Black plays Kf8 and now the White bishop is attacked twice and defended once with nowhere good to go. Remember that Black's rook would be on d5....) What then would be possible? White’s queen and bishop would be set up for the pawn fork b7-b5, with Black’s rook at b2 furnishing the cover.