Now our present theme seen from Black’s side. The sight of White’s bishop on a diagonal with his king is suggestive. The bishop isn’t a good target for a pin, but perhaps an exchange would fix that. Black has one piece usable for the purpose: he plays RxB, and just as in the previous positions White has to play RxR, rather than KxR, because d3 still is attacked. Now Black has the pin Bb5. White’s rook finds itself attacked twice and protected once, so White’s king won't be able to recapture when Black plays BxR on the next move.

Notice a recurrent theme: when the pinned piece is adjacent to its king and protected by it, try adding more attackers. The king’s protection of the piece then becomes illusory, since it can’t recapture on a square that still is under fire.