There is optionally the vocalisation of the semi-vowel śvi before the affixes of the Perfect and the Intensive.,
The phrases लिड् यङोः and सम्प्रसारणं are to be read Into this sutra. Thus शुशावे or शिश्वाय, शुशुवतुः or शिश्वियतुः ॥ So also in the Intensive as शीशृयते or शश्वीयते ॥ The root श्वि would not have taken vocalisation before यङ् by any previous rule, this sutra teaches optional vocalisation. The root would have taken vocalisation before लिट्, which is a कित् affix, invariably by 6.1.15, this sutra modifies that by making the substitution optional. In the alternative, when the root does become vocalised, the reduplicate is also not vocalised in spite of 6.1.17. This explains the form शिश्वाय, which by 6.1.17 would have been शुश्वाय ॥ श्वि + णल् = (श् उद्व + णल् 6.1.30) = शु + णल् 6.1.108 = शु + शु + अ 6.1.8 = शुशाव ॥,