The affixes śatṛ and śānac are substitutes of laṭ (Present Tense) when agreeing with what does not end with the 1st (Nominative Case) case-affix.,
As पचमानं देवदत्तं पश्य 'behold Devadatta who is cooking'; पचता कृतम् or पचमानेन कृतम् 'done by one who is cooking'. पच् + शानच् = पच् + शप् + शानच् 3.4.113, 3.1.68 = पच् + अ + मुक् + आन 7.2.82 = पचमान । So also पच् + शप् + शतृ = पच् + अ + अत् = पचत् ।
Why do we say 'when agreeing with what does not end with the first case-affix' ? Observe देवदत्तः पचति 'Devadatta cooks'.
Since the term लट् might have been supplied from the last aphorism, its double citation is for the sake of larger application of this rule; that is to say, this rule applies sometimes even when the word is in concord with a nominative case. As सन् ब्राह्मणः 'who is a Brahmana' = अस्ति ब्राह्मणः; so also विद्यते ब्राह्मणः and विद्यमानो ब्राह्मणः; जुहुत् and जुहोति; and अधीयानः and अधीते ।
Vart:- These affixes are employed after a verb, when the particle माङ् is in composition, and censure or curse is implied. As मा पचन् and मा पचमानः 'may he not cook'; मा जीवन् यः परावज्ञादुःखदग्धोऽपि जीवति 'cursed be he (lit. may he not live) who though smitten by the pain of the contempt of others, still lives'. Some commentators read the anuvritti of the word 'option' from sutra 121 into this. That option, however, is a limited option (vyavasthita-vibhasha) and not of universal application. Whenever a particular form is found as differing from the general form, there it should be understood that the option has been allowed, and not everywhere.,