अस्यतितृषोः क्रियाऽन्तरे कालेषु

The affix णमुल् comes after the verbs 1. अस् 'to throw' 2. तृष् 'to be thirsty' when it is intended to express an intermission of the action denoted by the root, provided that a word in the Accusative -2nd case denoting time is in composition.
The word क्रियान्तर means 'the interval between two actions'. Thus द्व्यहात्यासं or द्व्यहमत्यासम्म् गाः पाययति 'he gives drink to the cows, after an interval of two days, i. e every third day'. So also द्व्यहतर्षं or द्व्यहं तर्षं गाः पाययति 'having kept the cows thirsty for two days, he makes them drink'. That is to say, 'having given them a drink today, he gives them another drink after an interval of two days'; &c.

Why do we say 'after the verbs अस् and तृष्'? Because the affix णमुल् will not be applied after other roots, though the sense be that of interval of time. Thus द्व्यहमुपोष्य भुङ्क्ते 'having fasted for two days, he eats'.

Why do we say 'when denoting an intermission of action'? Observe अहरत्यस्येषून् गतः; here there is no intermission in the action of 'going'.

Why do we say 'denoting time' ? Observe योजनमत्यस्य गाः पाययति 'allowing an interval of four miles to pass, he gives drink to the cows i. e. he waters the cows at every four miles'. Here the interval is that of 'space' and not of 'time', and hence the affix is ल्यप् and not णमुल् ।