White does not yet have the makings of a discovery, but he can create the kernel of one by moving his rook into his bishop’s path. Both pieces are aimed at the square in front of Black's king, so the rook's move to g7 gives check and may create a windmill. May, and does. In the previous positions the trapped king had to shuffle between two squares because all others were blocked by the side of the board or by its own pieces. Here Black’s king is hemmed in by White’s knight on d6, which cuts off e8 and f7. So when White moves his rook from g7 and discovers check, Black’s king will have to go to g8; and when White moves his rook back to g7, Black’s king will have to return to f8.
The only question is what White should do with his rook to make the most of this pleasant opportunity. He starts with the familiar destruction of the seventh rank: Rxd7+ (Black plays Kg8), Rg7+ (Black plays Kf8). Now what? Rb7+ (Black plays Kg8); and now RxR+, winning a rook and applying another check from a different angle. Black has no escape from it; all his flight squares are attacked. All he can do is interpose Nf8, but then RxN#. We see again the importance of remembering to look not just for material but for mate.