وَالْمَسْحُ عَلَى الْخُفَّيْنِ عَلَى ظَاهِرِهِمَا خُطُوطًا بِالْأَصَابِعِ، يَبْتَدِئُ مِنَ الْأَصَابِعِ إِلَى السَّاقِ.
Meaning: The khuffs are wiped on their upper surfaces in lines with the fingers, beginning at the toes and moving toward the leg.
The prescribed area is the upper outer surface of each khuff, not the sole. The described Sunnah method moves the fingers from the toes toward the leg.
Making visible finger-lines describes the recommended manner; those lines are not themselves the minimum obligatory amount.