s is the substitute of a visarga before an affix beginning with a hard guttural or labial.,
The word अपदादौ means \when the guttural and labial are not at the beginning of a word,\ in other words, when they stand at the beginning of an affix. This is possible only before the affixes पाश, कल्प, क, and काम्य ॥ Thus पयस्पाशम् 5.3.47; पयस्कल्पम्, यशस्कल्पम्, 5.3.67, पयस्कं, यशस्कम्, 5.3.70; पयस्काम्यति, यशस्काम्यति (III. I. 9).
Why do we say 'when not at the beginning of a word'? Observe पयᳶ कामयते, पयᳶ पिबति ॥
Vart:- Prohibition must be stated, when the visarga belongs to an Indeclinable: as, प्रातः कल्पम्, पुनः कल्पम् ॥
Vart:- The visarga which comes from रु is only changed to स before काम्य, and not any other visarga. As पयस्काम्यात, and यशस्काम्यति; but not here, गीः काम्यति धूः काम्यति ॥
Vart:- स is the substitute of the Upadhmaniya when followed by a guttural. The root उᳶज् (आजवे Tudadi 20) has Upadhmaniya as its penultimate: though it is written in the Dhatupatha as उव्ज the व् only represents the प् of ᳶप, and is not to be pronounced. This ᳶप is changed to स, when the final ज् is changed to a guttural, as उ स ग, and then this स् is changed to द्, as in अभ्युद्गः, समुद्गः ॥
These words, however, may be derived from the root गम with the Prepositions अभि, उत्, and सम् उत्, by adding the affix उ ॥,