Figure 2.5.4.5[Black to move]

Here is a nice extension of the principle we are studying. Black examines every check as a matter of course, and finds two: Qe2 and Qd3. Qe2 loses the queen without compensation. But Qd3+ forces White to play QxQ, to which Black replies e4xQ. Now reexamine how the board would look: the pawn on d3 would be attacking the White knight on c2; and if the knight moves, the pawn advances to d2, forking both rooks. Could it then be taken by the king? No, because the knight formerly on c2 will have moved, creating an open line for the Black rook on b2 to provide cover for the pawn. So White is better off letting his knight on c2 get taken by Black’s pawn rather than letting the pawn march farther and take a rook.

Think of this as a case where a pawn again marched diagonally with a capture, enabling it to deliver (or threaten) a fork that would not have been possible on its original file. But in order to move over a file, the e4 pawn needed White to put something on d3 that it could take; the something—White’s queen—was drawn into place with a check (Qd3+).