Figure 6.2.7.4[White to move]

There are some conditions necessary before our current mating idea can work. Black can't have any good interpositions on the diagonal leading to g8, and his king can't have h7 available as a flight square. In this position White’s knight is enormously well-placed on a hole created by Black's advances of his f-pawn and h-pawn. The advanced f-pawn also suggests the possibility of mate using a bishop directed at f8. White can play Bc4+ easily enough, but now must consider the issues just mentioned. Can Black interpose his rook from e8? Not effectively: if he plays Re6, the rook is loose and just gets taken with BxR+, renewing the threat. (If Black’s rook were on f8 and able to interpose at f7, White’s mating threat would fail—though he still could win material.) But now what about h7? It's open as a flight square for Black’s king—but notice that White has a queen aimed at the square. So if Black plays Kh7 White can discover check by moving his knight. How to capitalize? Like so: 1. Bc4+, Re6; 2. BxR+, Kh7; 3. Nf8++, Kh8; Qh7#. The double check by the knight forces Black's king to move, and then the knight provides cover for White's queen on the mating square (h7). The larger point: the open h7 square meant that an additional piece was needed to execute the mate.