आडजादीनाम्

आट् acutely accented (उदात्त) is the augment of a verbal अङ्ग (stem) beginning with a vowel in the Aorist, Imperfect and the Conditional.
Thus ऐक्षिष्ट, ऐक्षत, ऐक्षिष्यत; औव्जीत्, औव्जत्, औव्जिष्यत् ॥ ऐहिष्ट, ऐहत and ऐहष्य औम्भीत्, औम्भत्, and औम्भिष्यत् ॥ The Vriddhi takes place by 6.1.90.

The Passive Imperfect (लङ्) of यज्, वप् and वह are ऐज्यत, औप्यत and औह्यत formed by आट् and not अट् ॥ First, the affixes of the Imperfect are added and then the Passive characteristic यक् is added to these roots, which causes the vocalisation of the semi-vovels, and we have इज्यत, उप्यत, and उह्यत stems 6.1.15. The stems having now assumed a form in which they begin with a vowel, take आट् ॥ The addition of tense-affixes being an antaranga operation precedes the addition of augment. After the affixes have been added, the vikarana यक, being nitya, is added and precedes in order the augment, the latter being so far anitya; after यक् addition the roots assume a form in which we can add आट् ॥ Why आट् is considered anitya depends on the following maxim :- शब्दान्तरस्य प्राप्नुवन् विधिरनित्यो भवति "when the word-form in reference to which a rule teaches something; after the taking effect of another rule that applies simultaneously would be different from what it was before that other rule had taken effect, then the former rule is not nitya."