अव्यक्तानुकरणाद्द्व्यजवरार्धादनितौ डाच्

The affix डाच् comes after the half of a word denoting an imitation of an inarticulate sound when such half consists of at least two syllables and when it is not followed by the quotation-mark इति।
The sound in which the letters अ आ &c are not distinctly expressed is called avyakta 'unmanifest' or 'inarticulate'. The imitation of such a sound is called avyakta-anukarana. That whose half consists of at least two syllables is called dvyajavarardha, that is a polysyllabic word of four syllables or more when doubled, the smallest number of whose vowels are two, not less, when halved. The phrase "when joined with kri, bhu and asti" is understood here. The affix comes after that word which when reduplicated is such that its half (ardha) is at least (avara) of two syllables (dvyach). When the affix डाच् is added there is reduplication of the word. That is first the word is reduplicated and then the affix is added. Thus पटत् is a sound imitation word. By reduplication it becomes पटपटत् (eg. पटत् + पटत् = पटप् पटत् the final त् of the first word is changed in to the form of the next letter प्, and then for two प one is substituted by 6.1.100). Thus पटपटत् करोति = पटपटा करोति "he makes a sound like patat patat". So also with भवति and स्यात् ॥ So also दमदमा करोति, भवति or स्यात् ॥ The final अत् of पटपटत् is elided by 6.4.143 before the affix डाच् having an indicatory ड्, the real affix being आ ॥

Why do we say "when it is name of an imitative sound?" Observe दृषत् करोति "it makes or turns to stone". Why do we say "after that half which consists of at least two vowels"? Observe श्रत् करोति 'he utters the exclamation srat'. Why do we say "at least"? Observe खरटखरटा करोति where the half consists of more than two syllables and the rule still applies. So also त्रपटत्रपटा करोति ॥ Why do we say "when इति does not follow"? Observe पटिति करोति 6.1.98. The च् of डाच् is for the sake of accent, it prevents the circumflex (swarita) accent. Thus, पटपटा꣡ + अ꣡सि ॥ Here first the अ of असि which was udatta, becomes anudatta by 8.1.28. Then by 8.2.5, आ꣡ of टा and अ of असि coalesce into an udatta आ꣡, as पटपटासि ॥ But then comes in rule 8.2.6 which would make this आ optionally svarita, the indicatory च् prevents it. Some read द्व्यजवरार्ध्यात्, the य however does not change the sense.