White has a single capture available with QxQ. It's prevented by Black’s rook on his back rank. White considers dropping one of his own rooks onto that rank, testing whether the Black rook there might be overworked. This time Black can move his king to g7—an option that, as we shall see, can complicate matters a bit. Here White’s recourse is clear enough, though: he then has RxR; and if Black replies QxR, White has QxQ. (Or if Black plays QxQ, it is met with RxQ.) The point: once your rook is on the enemy back rank, be broad-minded in thinking about what it might do there.