Once the pawn reaches the sixth rank, the defending rook must leave that rank and go behind the enemy king to deliver endless checks. The pawn itself takes away the king's hiding square.
Black has set up Philidor's position: the king on e8 in front of the pawn, the rook on b6 keeping the white king off the sixth rank. White's only attempt is 1.e6, and the student must now switch defensive methods.