Figure 4.3.3.9[White to move]

The Black king’s position may look reasonably secure. It isn’t. White considers any checks he can give—in this case, just one: Qxh7. Black has to reply KxQ. Trace the lines away from the new position you are imagining for the king and see that the pawn on g6 now would be pinned. That means the two squares the g6 pawn used to protect now are free. White can plant a piece on one of them with Rh5#, which mates since g7 is off limits to Black’s king. A simple case of a queen sacrifice drawing the king into line with a pawn—a measure justifiable as a means to mate.