The principle here is effective opposition. In short, black has the opposition here across 3 ranks and 1 file, but it won't reduce to an actual opposition across 1 square because of the f4 pawn's control of e5- this means that white can allow black to take the diagonal opposition:

1.Kc4! Ke6 (Kd6 2.Kd4 Ke6 3.Kc5+-)
2.Kc5!

And black can't take the opposition due to white's control of the e5 square.