Here is the same principle in slightly different form. What Black pieces are loose? The knight at a7 and bishop on d7. White can’t give check and attack either piece at the same time, so he asks whether he might attack one of them while creating a mate threat. He looks for pieces he has trained on squares next to the king and sees that the bishop at a1 is aimed at h8. If White’s queen were to land on h8 the game would be over. So White just needs a square from which his queen can (a) attack one of Black's loose pieces, and (b) attack the h8 square. Qd4, winning the knight, is the answer.