The ṣ substitution does not take place in the affix sāt and for that s which stands at the beginning of a word.,
Thus सात् here is the affix ordained by 5.4.52, and it would have been changed to ष by 8.3.59. The present sutra prevents that. The Padadi स refers to those verbs which in the Dhatupatha are taught with a ष, and for which a स is substituted. This also is an exception to 8.3.59. Thus अग्निसात् । दधिसात् । मधुसात् ॥ So also with Padadi स as दधि सिञ्चति, मधु सिञ्चति ॥,