Black would like to promote that f-pawn. The problem is that if Black plays f2 then White rolls the Rook to g7 or g8 to skewer the Queen if Black dares to promote. So, what did Black do to win this game?

Clever Chess

Result: 0-1
[…] 1…f2 (1…♗b1 2.♖xf3+ ♔xf3 draws) 2.♖g7 White hopes to skewer the Black King against the pawn that would promote to Queen … for example f8=Q then Rf7 check and the Queen falls… 2…♗b1 the clever trick 3.♔xb1 (3.♖f7+ ♗f5 4.♔xb3 f1=♕ and pawn Queens without any problem) 3…f1=♕+ 4.♔b2 ♕e2+ and the Queen is saved. Black can hope to win with extra material now