Nyaya - The slap being proportionate to the cheek.

यत्रसम्मुखं तत्रसचपेतैति न्यायः

It is used to denote that a measure must be taken for controlling one must be sufficiently strong in consideration of the particular case in question, just as the slap administered to a man must be tight enough in consideration of his physique and strenght to produce the intended effect.