Against a knight pawn (or a rook pawn) on the sixth, the defender can simply keep the king in front of the pawn and the rook on the back rank. The attacker never gets both the seventh rank and a safe square for his king.
White has everything the attacker usually wants: king on c6 next to the pawn, rook on the seventh rank. Against a bishop or centre pawn this would be lost, because White would threaten to check on the back rank and then promote. Against a knight pawn, the passive defence holds.