White’s attention to the Black king’s lines turns up the queen on d4, ready to be pinned on the long dark-squared diagonal. White’s dark-squared bishop is prepared to do the job by moving to c3. But a piece that pins the queen needs protection, and at c3 the bishop would have none; Black would merely play QxB. Can protection be arranged? What White needs are forcing moves that not only create the needed protection but keep Black too busy to escape the coming pin. Checks are best for this purpose, and White’s queen is in a position to check Black’s king easily. The concept becomes clear: think backwards about how the White queen can check its way onto g3, where it then would protect the pinning square, c3. He starts with Qh4+, which requires Black to spend a move playing Kg8. Then White has Qg3+, which requires Black to spend more time moving his king back to h8 (this reconstructs the kernel of the pin by putting Black’s king back in line with his queen). Now that White’s queen has worked its way down to the third rank, he safely can play the pin Bc3, winning Black’s queen for a bishop.