Bhashya
स एतां पुनरभ्यतपत् त्रयीं विद्याम् ।तस्यास्तप्यमानाया रसं भूरिति व्याहृतिं ऋग्भ्यो जग्राह ; भुवरिति व्याहृतिं यजुर्भ्यः ; स्वरिति व्याहृतिं सामभ्यः ।अत एव लोकदेववेदरसा महाव्याहृतयः ।अतः तत् तत्र यज्ञे यदि ऋक्तः ऋक्संबन्धादङ्गनिमित्तं रिष्येत् यज्ञः क्षतं प्राप्नुयात् , भूः स्वाहेति गार्हपत्ये जुहुयात् ।सा तत्र प्रायश्चित्तिः ।कथम ? ऋचामेव, तदिति क्रियाविशेषणम्, रसेन ऋचां वीर्येण ओजसा ऋचां यज्ञस्य ऋक्संबन्धिनो यज्ञस्य विरिष्टं विच्छिन्नं क्षतरूपमुत्पन्नं संदधाति प्रतिसंधत्ते ॥
In plain words
He meditated once more upon the threefold Veda. As it was heated, he gathered the utterance 'Bhūḥ' from the Ṛg verses, 'Bhuvaḥ' from the Yajus formulas, and 'Svaḥ' from the Sāman chants. Hence these great mystical utterances are the very essences of the worlds, deities, and Vedas. Therefore, if the sacrifice suffers a flaw on the side of the Ṛg hymns due to an auxiliary failure, one should offer an oblation into the Gārhapatya fire saying 'Bhūḥ, svāhā!'. That serves as the expiation. How does it work? By means of the very essence, potency, and strength of the Ṛcs, he heals and restores the broken, damaged portion of the sacrifice.
Close to the Sanskrit
He concentrated again on this threefold Veda. From it being heated, he grasped the utterance 'Bhūḥ' from the Ṛg verses; the utterance 'Bhuvaḥ' from the Yajus formulas; the utterance 'Svaḥ' from the Sāman chants. Therefore indeed the great vyāhṛtis are the essences of worlds, deities, and Vedas. Hence, if in that sacrifice the sacrifice should suffer a flaw from the Ṛc side due to a connection with Ṛc, one should offer in the Gārhapatya fire with 'Bhūḥ svāhā!'. That is the expiation there. How? Through the essence of the Ṛcs alone, by the potency of the Ṛcs, by the might of the Ṛcs, he joins together and restores the broken, injured, arisen part of the Ṛc-related sacrifice.