धात्वादेः षः सः

There is the substitution of स in the room of the ष् being the initial of a verbal root as enunciated in the धातुपाठ।
The roots exhibited in the Dhatupatha with an initial ष, change it for स ॥ Thus षह -सहते, षिच् - सिंचति ॥ Why do we use the word 'root'? Observe षोडश, षडिकः, षण्डः ॥ Why do we say 'initial'? Observe — कर्षति, कृषति ॥ Why have then roots been exhibited in the Dhatupatha with an initial ष, when for all practical purposes this ष is to be replaced by स, would it not have been easier to spell at once these words with a स ? This appears cumbersome no doubt, but the spelling of the roots with ष is for the sake of brevity. Certain roots change their स into ष when preceded by certain letters: those roots which thus change their letter स for ष have been at once taught with an initial ष, and thus by 8.3.59 their स is changed to ष ॥ Thus from सिव - instead of सिसव we have सिषव ॥ Thus root is exhibited in the Dhatupatha with a ष, and thus we know that the स must be changed to ष ॥ Otherwise a list of such roots would have to be separately given. One must refer to the Dhatupatha for a list of such roots. As a general rule, however, all roots beginning with a स and followed by a vowel or a dental letter have been taught in the Dhatupatha as beginning with an initial ष, as well as the following roots स्मि, स्वद्, स्विद्, स्विज् and स्वप्, though followed by म् or व् ॥ The following roots though followed by a vowel or a dental are not taught with an initial ष viz. सृप्, सृज्, स्तृ, स्त्या, सेकृ and सृ ॥

Vart:- Prohibition must be stated of the roots derived from nouns, and of ष्ठिव् and ष्वष्क ॥ Thus षोडीयते, षण्डायते are roots derived from nouns, and though these roots begin with ष are not changed: so ष्ठिव् — ष्ठिवति, ष्वष्क — ष्वष्कते ॥ The word ष्ठिव contains in it two roots ष्ठिव् and ष्थिव्, one with ठ, another with थ, and thus we have two forms in the reduplication :- तेष्ठीव्यते, टेष्ठीव्यते ॥ The substitution of स for ष takes place in the case of the root that has थ ॥