Figure[Black to move]

Black doesn't have two pieces aimed next to White’s king, but when you study the king’s position you see that it has another problem: it's trapped on the back rank and has no defenders there. Black almost can mate with either Rc1 or Qe1. Both possibilities are prevented by White’s queen, which protects the two mating squares. Next question: so what else does the White queen purport to do? It guards the rook on g2; but really the rook is as good as loose since the queen cannot afford to leave d2. Black takes it with his queen.