1.g4 clears g2 for the rook and threatens 2.Rg2+ Kf1 3.Ke3 any 4.Rg1. After 1...Bc6 2.g5 clears g4 to threaten 3.Rg4 with a fatal check at f4 to follow. If 2...Bd7 3.Rg2+ as before, while if 2...Bf3 3.Rg3 forces the bishop to move (3...Bh5 4.Rg2+ or 3...Bc6 etc. 4.Rg4). Black’s best defence is 2...f5, after which 3.gxf6 e.p. renews the threat of 4.Rg4. After 3...Bf3 4.f7 threatens to queen and capture on f3, while a small surprise follows 4...c5 when only 5.f8B any 6.Bxc5 works. Of course promotion to bishop is also a threat at move 5. The idea of excelsior with underpromotion is not obvious in the diagram.