अवयवादृतोः

Before a तद्धित affix having an indicatory ञ् , ण् or क् वृद्धि is substituted for the first vowel of a word denoting season when it is preceded by a word denoting a part.
As पूर्ववार्षिकम्, पूर्वहैमनम्, अपरवार्षिकम्, अपरहैमनम् ॥ The composition with पूर्व and अपर &c takes place by 2.2.1. and then by 4.3.18 there is ठक् after वर्ष, and अण् after हेमन्त with the elision of त by 4.3.22. The Tadanta-viddhi, as a general rule, does not apply to compounds, so that a rule made applicable to a particular word, will not apply to a compound which ends with that word: but tadanta-viddhi applies to a word denoting season when it takes an affix causing Vriddhi, and is preceded by a word denoting a portion. We draw this rule from the present sutra, for हैमनं being formed from हेमन्त by a vriddhi-causing affix 4.3.22, the affix अण् will be applied to हेमन्त even when it is the second member of a compound, the first member of which denotes a part. (ऋतोर्वृद्धिमद्विधाववयवात् ( I.1.72) Vartika Mahabhashya).

Why do we say 'denoting a portion'? Observe पूर्वासु वर्षासु भवं पौर्ववर्षिकम् with ठञ् 4.3.11. The tadanta-vidhi applies only when the first member denotes a portion.