Now let's consider pawn discoveries that require preliminary exchanges. Our focus will be on a single motif: creating a discovered attack in which the pawn unmasks a diagonal attack by a bishop or queen in the direction of the enemy king. Afterwards we will look at a few pawn discoveries that follow other patterns.
What are the critical visual facts of the position to the left? They are, first, that White’s pawn masks his bishop (the formation on b2 and c3 should strike you right away); and, second, that White’s bishop and queen both are aimed at the square in front of Black’s king. Since White’s pawn can attack Black’s queen with c3-c4, the important question becomes the target at the other end—on g7. If White plays c3-c4 now, he also threatens mate with QxN. Sometimes a mate threat is as good as a check, but not always, and not here; the chief difference between them is that a mate threat gives the enemy a wider choice of replies. If he can inflict a check of his own—especially with his threatened piece—he may be able to put out the fire without doing anything about his king. That is the trouble in this case: after c3-c4, Black has the check Qd1. Now White is the one whose move is forced. He must play his king to the second rank. And then comes the next check: Qd2+, a double attack on White’s king and his loose bishop. White can avoid the loss of the bishop with Qf2, but this wasn’t exactly what he had in mind at the outset.
Meanwhile, of course, there is a better alternative to all this. Instead of playing c3-c4 first, White starts with the preliminary exchange 1. QxN, KxQ. Now c3-c4+ is a discovered check, giving Black no chance to take the offensive and winning back the queen for White (with the gain of a piece) a move later.