There are only two ways for Black to defend f7. One is to advance
the Queen's Pawn to d5, interrupting the diagonal of White's
Bishop; the other is to castle, so that the Rook procures the
second protection for the Pawn f7 which is needed. It would then
not be good for White to capture the Pawn because he would have
to give up Knight and Bishop for Rook and one Pawn, which is not
a sufficient equivalent.

Of the two ways indicated only the first one is feasible. For if
Black castles he gives White an opportunity to institute an
attack on the weak Pawn h7 with Knight and Queen against which
Black has no satisfactory defense. Play would develop as follows:

(1) Kt-g5 o-o
(2) Q-h5 P-h6

This is the only defense against the threat Qxh7 mate. But
White's Queen's move involved a double threat. It brought up a
third attack on the Pawn f7, and the latter now falls, forcing
Black to give up some more material.

(3) Ktxf7 Rxf7

Black has to sacrifice the exchange. If he moved the Queen, which
is attacked by the Knight, he would expose his King to a deadly
double check, viz.: (4) Ktxh6++, K-h8 or h7; (5) Kt-f7+
(discovered), K-g8; (6) Q-h8 mate. After giving up his Rook for
the Knight on the third move Black has a lost game, for as
explained in the previous chapter White can simply exchange all
pieces and force the win in the ending with his superior