The augment am (a) comes after the ṛ or sṛj 'to create' and dṛś 'to see', when an affix beginning with a jhal letter (the Mute and the Sibilant) follows provided that it has no indicatory ka, provided that it has no indicatory ka,
Thus सृज् + तृन् = सृ + अ + ज् + तृ = स्रष्टृ= nominative singular स्रष्टा, so also स्रष्टुम्, स्रष्टव्यम्, द्रष्टा, द्रष्टुम्, द्रष्टव्यम् ॥ This अम् augment prevents the guna substitute ordained by 7.3.86. But in अस्राक्षीत् and अद्राक्षीत् the Vriddhi takes place in the Aorist, after the augment अम् had taken effect.
Why do we say when beginning with a Mute or a Sibilant? Observe सर्ज्जनम्, दर्सनम् with यु ॥ Why do we say not having an indicatory क? Observe सृष्टः, दृष्टः before the affix क्त ॥ The forms of roots being exhibited in the sutra, the rule applies when affixes relating to verbs come after these, and not when other affixes relating to noun follow. Thus रज्जुसृङ्भ्याम्, देवदृग्भ्याम् ॥ Here the words are used as nouns rather than verbs. For the maxim धातोः स्वरूपग्रहणे तत्प्रत्यये कार्ये applies here.,