Now a still harder one. White already has the Black bishop pinned on the eighth rank. If he takes it with his queen, the queen is lost to QxQ. Ne7+ is the more interesting move, because it attacks the pinned piece and also gives check. If Black moves his king in reply to Ne7 (playing Kf8), he loses the bishop since it is attacked twice and protected just once. So he needs to capture White’s knight. Could he do it with NxN? No, because if you imagine the board with both of those knight moves made you will see that they would open a line from White’s bishop to Black’s queen, allowing BxQ. (In effect, White’s move Ne7 leaves Black’s knight pinned to his queen.) The remaining alternative for Black is QxN, which allows QxB+—apparently an exchange of minor pieces. But White’s capture of the bishop also imposes a check on Black’s king, so we must consider the consequences. The king can’t flee to h7 because White’s bishop guards the square. Black has no choice but to interpose his queen on the back rank. The only way for him to do this is with Qf8, since then the queen enjoys protection from the king; moving the queen instead to e8 or d8 results in QxQ#.
Now what? The point to recognize is that by interposing in this way, Black’s queen has pinned itself. It is attacked once and protected once, and so seems safe; but since its only guard is the king, Black is highly vulnerable to a check. White has one piece left for the purpose: he plays Bh7+. Black has no choice but to play KxB—which leaves the queen loose. White takes it on the next move.