When your rook is stuck in front of an enemy passed pawn, your king must race to the pawn. If it arrives in time to cover the promotion squares, the position is drawn.
The mirror of the previous lesson: Black's rook is passively placed on a8 in front of the a-pawn, White's rook is actively behind it, and both kings start running. The black king is on g7 this time, exactly one tempo better than in the winning version.