Figure 4.2.1.4[White to move]

White scans the lines leading to Black’s king and sees that the knight on c6 is ready to be pinned; he looks for a resource he can use on the knight’s diagonal and finds Qa4. This move also has the virtue of attacking Black’s bishop on a6, which is loose. The bishop both moves to safety and provides protection for the knight with Bb7, so White won't be playing QxN. But now comes the pawn push: d4-d5, and the paralyzed knight is lost irrespective of how much protection it has. This pattern—a pawn in the middle of the board marching forward to take a piece pinned to its king on the enemy’s third rank—is common enough to be worth a moment's study.