Before an iṭ beginning substituted-Aorist of the parasmaipada there is optional vṛddhi of the vowel of ūrṇu,
As प्रौर्णावीत् or प्रौर्णवीत् ॥ This option applies when the सिच् is not treated as ङित् ॥ But after ऊर्णु, सिच् is optionally ङित्, 1.2.6: when it is ङित्, there being neither guna nor vriddhi; we have उवङ् substitution, as प्रौर्णुवीत् 6.4.77.,