ऋतो भारद्वाजस्य

In the opinion of भारद्वाज , it is only after a root which ends in short ऋ and after which the Periphrastic Future तास् is always devoid of the augment इट् , थल् the personal-ending of the लिट् (Perfect Tense), like तास् does not get the augment इट् ।
As स्मर्त्ता, सस्मर्थ, ध्वर्त्ता, दध्वर्थ ॥ The ऋ roots are covered by 7.2.61; the specification of these roots makes a restriction, so that all other roots, are not anit, though the Periphrastic Future in तास् be anit. So that according to Bharadwaja, we have forms like ययिथ, वयिथ, पेचिथ, शेकिथ ॥ Thus this sutra makes the preceding two sutras optional, except so far as short ऋ-ending roots are concerned.

As a general rule, all roots ending in long ॠ are सेट्; therefore, the sutra mentions short ऋ, in order to debar its application to long ॠ ending words. Had it not been so, this rule would have been a vidhi rule and not a niyama: because rule 7.2.61 does not apply to long ॠ ending roots, and so if the present sutra were to include long ॠ also, it could not be called a niyama (restriction) of 7.2.61.