White’s bishop masks an attack up the d-file; this time the potential attacking piece is White’s queen. White’s bishop has an obvious place to go that will cost Black some time: Bxh7+. The problems are that Black’s queen—the natural target of the operations—is guarded by a knight, and that the Black pawn on d4 blocks the d-file. Start with the second problem; examine how the pawn is threatened and defended. It is attacked by White’s c-pawn and two knights, and defended by a knight, a bishop, and a queen. Play through the liquidation of those pieces in your mind’s eye and see what is left at the end: 1. c3xd4, Nxd4; 2. NxN, BxN; 3. NxB, QxN. With the board thus simplified, what would be possible? The c and d pawns would be gone; both of White’s knights would be gone; Black’s knight and bishop would be gone; and Black’s queen would be loose on d4, in front of White’s bishop—a classic setup for a discovered attack via Bxh7+. After Black plays KxB, White plays QxQ, winning Black’s newly loosened queen.
This position looks a little complicated because it involves several exchanges, but its structure is simple. Once you see the formation for a discovered attack, you just methodically work through all the exchanges bearing on the obstructions that prevent it from working. This example illustrates a particularly useful version of the idea: when an enemy pawn blocks a discovered attack against a piece that lies behind it, perhaps the piece you want sooner or later can be made to replace the pawn if the pawn is taken. That is what happened here: the d4 square never was emptied; rather, the pawn eventually was replaced by the target of the exercise—the queen. But to get to any of this you first have to see the basic pattern for a discovery on the d-file without being blinded by the obstacles in the way.
Of course if Black sees all this coming he will simply forfeit the pawn at the beginning rather than head down the road toward larger losses by making a recapture; he will notice the kernel of a discovery for White and will realize that d4 is a square of danger.
This position is a good study in the value of playing through a series of liquidations in your mind's eye. The particular type of liquidation shown here is fairly common: a pawn near the middle of the board often will be protected and defended several times, making it important to understand what would happen if all those potential captures and recaptures were played out.