The affix ṇini comes after a verb when in composition with a word denoting an object of comparison, expressing the agent, the sense of the affix being 'doing something like that', that is to say, when the upapada agent is the standard of comparison of the agent denoted by the word to which the sense of the affix refers.,
Thus उष्ट्र इव क्रोशति = उष्ट्रक्रोशिन् 'who makes a noise like a camel'; ध्वाङ्क्षराविन् 'who makes a noise like a crow'.
This sutra applies where 'habit' is not indicated : or where 'genus' is denoted in opposition to the last aphorism.
Why do we say 'when expressing the agent'? Witness as अपूपानिव भक्षयति माषान् 'he eats masha like apupa'. Here अपूपा is object and not agent, and hence no composition takes place.
Why do we say 'denoting a standard of comparison'? Otherwise there will be no composition. As उष्ट्रः क्रोशति 'the camel makes noise'. For the accents of these words, see 6.2.80.,