ṣ is substituted for the s in sāha when this occurs in the form of sāḍa (sāṭ).,
Thus जलाषाट्, तुराषाट्, पृतनाषाट् ॥ साड् is derived from सह् by the affix ण्वि 7.2.63, there is vriddhi of the penultimate, the ह is changed to ढ 8.2.31. and the upapada is lengthened 6.3.137.
साढः सः would have been enough, for there is no other form साड् except this derived from सह्; why then the word सहेः is used in the sutra? There is another form साड् not derived from सह् ॥ Thus सह डेन वर्त्तते = सडः, सडस्य अपत्यं = साडिः ॥ He in whose name there is the letter ड is called सड; as मृड ॥
Why do we say 'in the form of साड्'? The rule will not apply when the form is साह, as जलासाहम्, तुरासाहम् ॥ Why do we say सः \for the स\? So that the आ of साड् may not be changed to cerebral : the ड् is already cerebral.,