The affix yañ comes, in the sense of a gotra (Patronymic) descendant after the words 1. madhu and 2. babhru when the words so formed mean brāhmaṇa and kauśika respectively.,
Thus माधव्यः = ब्राह्मणः 'Brahmana.' Otherwise माधवः \son of Madhu.\ बाभ्रव्यः = कौशिकः Otherwise बाभ्रवः ॥
The word बभ्रु belongs to the Gargadi class, so it would have taken yan by the last sutra also. The present sutra declares a restriction, viz. बभ्रु takes यञ् only then when कौशिक is denoted. Its being read in the Gargadi class serves another purpose : as it belongs to the sub-class लोहितादि, the rule 4.1.18 becomes applicable to it; so that its feminine is formed by ष्फ. Thus बाभ्रव्यायणी ॥ In fact, it would have been better, if in the body of the Ganapatha, instead of writing बभ्रु alone, there was बभ्रु कौशिके ॥ It would have saved the repetition of it in the present sutra.,