The only loose Black piece is the bishop at a1. White can’t attack it and give check at the same time, so he looks for mating threats. His knight attacks f7 and h7; the attack on f7 is not powerful because Black protects the square with his rook as well as his king, but h7 is only protected by Black’s king. It follows that if White’s queen attacks h7 it will threaten mate; the h7 square thus can be treated a target in the same way a loose piece or the king itself would be. So the question for White is whether his queen can attack the Black bishop and the h7 square at the same time. It can; Qh1 threatens mate and so wins the bishop. The long backward move of the queen here is counterintuitive, and illustrates the importance of methodically looking at every way the queen can attack the vulnerable points in the enemy position.