The White's King is locked on g8 with the Rook controlling h-file, and King from the other side. White needs to push away one of the blockers, for the King to move.1.Re1+Kd71...Kd8Makes it even easier.2.Re5Kd73.Kf7Rf2+4.Kg6Rg2+5.Rg5White promotes on the next move.2.Re4...Has a deep idea, which you will understand in a moment!2.Kf7Gets under a lot of checks.Rf2+3.Kg6Rg2+4.Kf6Rf2+5.Kg5Rg2+White's King can't go lower ranks as the pawn on g7 will be lost, so it has to go back to g8.2....Rh13.Kf7Rf1+4.Kg6Rg1+5.Kf6Rf1+5...Rg2Black can try to wait on the g-file, what would you do?6.Re5Rg17.Rg5Again covering g-file and promoting the pawn.6.Kg5Rg1+Da-dam!!7.Rg4And White wins! This powerful technique is called "building the bridge". The White Rook covers the g-file and the pawn promotion is unstoppable.