The iṭ is optionally added to endings of the ātmanepada Benedictive and s-Aorist, after a root which ends in short ṛ which is preceded by a conjunct consonant.,
As ध्वृषीष्ट or ध्वरिषीष्ट; स्मृषीष्ट, or स्मरिषीष्ट; अध्वृषाताम्, or अध्वरिषाताम्; अस्मृषाताम्, or अस्मरिषाताम् ॥ ऋत इति किम् । च्योषीष्ट, प्लोषीष्ट; अच्योष्ट, अप्लोष्ट ॥ संयोगादेरिति किम् । कृषीष्ट, हृषीष्ट; अकृत, अह्रत ॥ आत्मनेपदेष्वित्येव । अध्वार्षीत्, अस्मार्षीत् ॥ संस्कृषीष्ट समस्कृतेत्यज्ञोपदेशाधिकारादभक्तत्वाच्च सुट् इडागमो न भवति ॥
Why ending in short ऋ? Observe च्योषीष्ट, अच्योष्ट, प्लोषीष्ट and अप्लोष्ट ॥ Why 'beginning with a conjunct consonant'? Observe कृषीष्ट, हृषीष्ट, अकृत and अहृत ॥ Why 'in the Atmanepada'? Observe अध्वार्षीत्, अस्मार्षीत् ॥ In संस्कृषीष्ट and समस्कृत there is not इट्, first because स्कृ (the form assumed by कृ with सुट् augment) is not so enunciated in the Dhatupatha; the word upadesa 7.2.10, is understood here; so that the rule applies to those roots only which in the Dhatupatha are ऋ ending and preceded by conjunct consonant; and secondly सुट् augment is considered as not attached to the root (VI. I. 135), and therefore स्कृ is not considered a root beginning with a conjunct consonant.,