Black cannot defend passively with a Rook on the back rank where White has any pawn other than a knight's or rook's pawn. If Black to move had his Rook on a2 a series of checks would easily defend.
1.Rb7
Rc8
2.Rg7+
Kf8
[2...Kh8
3.Rh7+
Kg8
4.f7+
]
3.Rh7
Kg8
4.f7+
Kf8
5.Rh8+
wins *