ru is optionally substituted for that final n of a word which is preceded by a long vowel and is followed by a letter of the aṭ pratyahāra (vowel or semi-vowel with the exception of l) when these (n and aṭ) come in contact with each other in the same stanza of the ṛgveda,
The नः of 8.3.7 is understood here : and so also ऋक्षु ॥ The word समानपाद means एकपाद, i. e. when both words are in one and the same Pada of the verse. Thus परिधीँ रति (Rig. IX. 107. 19) स देवाँ एहवक्षति (Rig. I. 1. 2) देवाँ अच्छादीष्यत्; महाँ इन्द्रो ओजसा ॥
Why do we say 'preceded by a long vowel'? Observe अहन्नहिम् ॥ Why do we say \when followed by a vowel or य, व or र'? Observe इभ्यान् क्षत्रियान् ॥ Why do we say 'when both words are in the same Pada of a verse'? Observe यातुधानान् उपस्पृशः ॥
The word उभयथा of the preceding sutra is understood here also : so that it is an optional rule : and न् remains unchanged also, as आदित्यान् हवामहे आदित्यान् याचिषामहे ॥ See 8.3.3.,