ṣ is the substitute of a visarga when is preceded by i or u and is followed by an affix beginning with a hard guttural or a labial.,
The word अपदादौ is understood here also. The affixes meant are the same पाश, कल्प, क, and काम्य ॥ Thus सर्पिष्पाशम्, यजुष्पाशम्, सर्पिष्कल्पम्, यजुष्कल्पम्, सर्पिष्कम्, यजुष्कम्; सर्पिष्काम्यति, यजुष्काम्यति ॥
Why do we say 'by an affix'? Observe अग्निः करोति, वायुः करोति, अग्निः पचति, वायुः पचति ॥
The affix should begin with a guttural or a labial. Therefore not here, सर्पिस्ते, यजुस्ते ॥
In the succeeding sutras, the anuvritti of स from 8.3.39 and of इणः षः from this, are both current. The visarga will be changed to ष if preceded by इ or उ, otherwise it will be स ॥
According to some, this sutra ordains ष् in the room of the स् taught in the preceding sutra, and not of visarga : and so also in the following sutras.,