Figure 2.1.8.4[White to move]

1. Ng6+ looks like a promising fork of Black's king and rook, impeded only by the knight at e7. One way to handle this, of course, is to capture the knight with 1. RxN, to which Black replies 1. …BxR. Now the fork Ng6+ wins back the exchange, but the sequence is a wash. So start again, taking a moment to look at what happens if you try for the fork without clearing g6 and just let the knight get taken. Notice especially the battery of two rooks on the seventh rank, which is always a powerful combination. If Black replies to Ng6 with NxN, that opens the seventh rank; what checks does White then have? Answer: just one; RxR―which is mate. So Black doesn’t dare play 1. ...NxN; in effect his knight is pinned to the mating square at h7. White’s best bet thus is to play 1. Ng6+ straightaway followed by NxN, as this wins a whole knight for free—a rare case where a knight fork wins a knight. If White instead finishes with NxR, he merely wins the exchange, as Black will then be in a position to recapture with his king (it would have moved to g7) whereas if White finishes with NxN Black can't recapture with his rook because his king is in the way.