After a word ending in ḍ there may optionally be added the augment dh to a word beginning with s,
Thus श्वलिट् त्साये or श्वलिट् साये, or मधुलिट् त्साये or मधुलिट् साये ॥
The word डः is to be construed as Ablative singular, and not Genitive singular of ड्, because of the maxim उभयनिर्देशे पञ्चमी निर्देशो बलीयान् ॥
Question:- Why it is धुट् and not धुक्, in other words, why is this augment added to the beginning of the second word and not to the end of the first ?
Answer:- This is done in order to prevent the ष्टु change by 8.4.41. But being at the beginning of the second word, 8.4.42 would prevent this change. श्वलिट्त् + साये = श्वलिट्ट्साये, 8.4.41 but the correct form is श्वलिट् त्साये ॥ For the ह of श्वलिह् is ढ substituted by 8.2.31, which becomes ड् by 8.2.39, for the ध of the augment, त is substituted by 8.4.55, and for ड् there is ट् by the same rule.,