Much more to this than first meets the eye ! Why Black promotes to a rook instead of a queen isn't immediately obvious until you see the fork: Rf5+Capture by a queen would result in stalemate (the White king is left with no legal moves) but by a rook will eventually win !
O/k, White should play 6.Kh8 to be as obstructive as possible but it's still a fun & instructive puzzle.
6. Kh8 Kg6 7. Kg8 R moves (except Rf8?? DRAW) 8. Kh8 Rf8#
The position is an endgame study by Scossi from either the 18th or 19th Century I think. Here is the full study:
Not much of a pluzzle, really
Completely missed it