While the black king is in the square and can snap up the white c-pawn anytime, the white king is out of the square, unable to catch the black pawn. Reti makes the study work, blending both squares into a beautiful solution.
1.Kg7!
h4
2.Kf6!
Kb6
3.Ke5!
(The diagonal journey is successful. The white king can either help his own pawn or catch the black one.)
3...h3
[3...Kxc6
4.Kf4=
]
4.Kd6
1/2-1/2