A look at the White king's lines turns up possible targets for a pin on the diagonal—but one too many of them. Removing White’s rook or queen from the board would leave the other piece pinned. If Black could take White’s queen, White would recapture with his king and the rook would be pinned; but that’s impossible. So consider the other end: can Black take White’s rook? Yes, with the pinning piece: QxR, leading to QxQ. Now White’s queen and king are alone on the long diagonal and the board is arranged for a pin. By using the queen to capture, of course, Black lost the pin that was in place at the start, but now he creates a fresh one with Bb7. The bishop pins the queen with protection from the knight on d6. White’s queen is lost, producing a net gain for Black. Lesson: sometimes you can consolidate excess enemy pieces by taking one of them with the pinning piece, then imposing a fresh pin once the line has been simplified to just the king and the target.