In a aṅga (stem) ending in a consonant with an a immediately preceding it, the vṛddhi is substituted for such a when followed by an indicatory ñ or ṇ,
As पाकः, त्यागः, यागः with घञ्, पाचि with the causative णि, पाचकः with ण्वुल् ॥ Why do we say 'अ'? Observe भेदयति, भेदकः with guna only from भिद् where इ is penultimate and not अ ॥ Why do we say 'penultimate'? Observe चकासयति, तक्षकः ॥,