As usual we find a bishop adjacent to its king on a diagonal. It can’t be pinned, but it's on a square that is subject to a pin. White asks whether he has a way to exert a pin against the bishop’s position; he looks for pieces that can operate on a diagonal and sees that Qa3 would work—if Black’s bishop were replaced by a piece that wouldn’t be able to bite back. So White looks for an exchange. It can be done with his rook or knight, so he picks the cheaper piece and plays NxB. Black replies NxN. Now comes Qa3 and the knight is immobilized. White can take the knight with either piece—his rook or queen—since the one not used will provide cover for the one that is, preventing Black’s king from recapturing.