Figure[White to move]

White’s knight on f3 appears to be pinned to his queen by Black’s bishop in familiar fashion. But we have seen that sometimes a pin of a piece against the queen can be turned into a discovered attack by the queen; this is a particular danger where, as here, Black has left the bishop loose in his eagerness to impose the pin. We just need a very threatening move for the knight. From f3 it can’t check Black’s king, but study the king’s position: once more White’s bishop bears down on it, attacking an adjacent square—the characteristically weak f7 (it's "weak" because only Black's king guards it). If White’s knight can’t give check, can it at least attack that square? Yes, with Nxe5, unmasking QxB.

Now assess White’s attack on f7 to see how serious it is. White’s next move—it would need to be a check, and there’s only one—is Bxf7+. In principle the king has three flight squares, but two of them (d7 and f7) are attacked, so Black’s only legal move will be Ke7. What is White’s follow up? Notice that now the king is severely boxed in; most of the squares around it are attacked. Again the important thing is to look for the next check—and to remember other pieces you can add to complete the net. Here the knight on c3 moves to d5, and it's mate.

What this analysis means is that Black has to respond defensively to 1. Nxe5, probably with d6xN. This allows White QxB and the net gain of a pawn.