Figure 3.3.4.3[White to move]

The kernel of the discovery is clear: White’s knight masks his queen. Where can the knight go, and what can the queen then attack? The general answer should be clear from the look of the board: the knight can head in the king’s direction, and then so can the queen; with a single move of the knight they both can attack h7. Meanwhile White’s rook already attacks that square. With three pieces ready to bear down on the territory of Black’s king, think in terms not of material but of mate. Consider Nf6; imagine each possible response by Black, and each check White would have afterwards. You will find that no matter what Black plays—e.g., NxQ, or g7xN, or RxN, or h7-h6, or g7-g6—White is then able to play either Qxh7# (supported by the knight or rook), or Rxh7# (supported by the knight). It's nevertheless important to notice all of those possibilities.

The point: you can use a discovered attack not only to win material but to mate.

You might have come at the position a bit differently. White is in a hole; he is down two pieces (though ahead two pawns). He needs to make something happen. The obvious focus of his attention is h7, where his rook and queen are aimed. A natural thought is Rxh7+, with the double check Nf6++ or Ng5++ to follow if Black replies KxR. But you don’t play a sacrifice like that unless you are sure it works, and as it turns out the double check doesn't end up achieving enough to save White. So you experiment with different move orders that likewise attack h7 and realize that Nf6 is not only strong but conclusive.