يَمْسَحُهُ حَتَّى يُنْقِيَهُ
Translation: He wipes with it until it cleans the area.
The words “until it cleans” tie completion to removal of filth. Merely passing a stone over the surface does not suffice while impurity remains.
This dry-cleansing permission concerns the ordinary exit region; a wider spread of impurity is treated separately.