ṣ is optionally substituted for the s of stan after the double upasarga (Preposition) abhi nis when the word so formed is the name of a particular letter (visarga).,
As अभिनिष्टानो वर्णः, अभिनिष्टानो विसर्जनीयः or अभिनिस्तानो वर्णः or विसर्जनीयः ॥ The compound preposition अभि-निस् causes this change, and not any one of them separately. Thus Apastamba :- द्व्यक्षरं चतुरक्षरं वा नामपूर्वमाख्यातोत्तरं दीर्घभिनिष्टान्तं, घोषवदाद्यन्तरन्तस्थम् \a name (nama) should be such that it should consist of two-syllables or four-syllables, that its first portion should be a noun (nama-purva), and its second portion a verb (akhyata), that it should end in a long vowel (dirgha) or a visarga (abhinishtana), that the first letter of such a noun should be a ghosha or sonant letter (ghosha-vad-adi), and a semi vowel (antastha) should be in the body of it.\ Thus the names द्रविणोदा, वरिवोदा fulfill these conditions and are good names.
Why do we say \when it is the name of a particular letter.\? Observe अभिनिस्तनति मृदङ्गः ॥ The anuvritti of समासे ceases from this place.,