Bhashya
सोऽकामयतेत्यश्वाश्वमेधयोर्निर्वचनार्थमिदमाह ।भूयसामहता यज्ञेन भूयः पुनरपि यजेयेति ; जन्मान्तरकरणापे-क्षया भूयःशब्दः ; स प्रजापतिर्जन्मान्तरेऽश्वमेधेनायजत ;स तद्भावभावित एव कल्पादौ व्यवर्तत ; सोऽश्वमेधक्रियाका-रकफलात्मत्वेन निर्वृत्तः सन्नकामयत— भूयसा यज्ञेनभूयो यजेयेति ।एवं महत्कार्यं कामयित्वा लोकवदश्राम्यत् ;स तपोऽतप्यत ; तस्य श्रान्तस्य तप्तस्येति पूर्ववत् ; यशो वीर्य-मुदक्रामदिति स्वयमेव पदार्थमाह— प्राणाः चक्षुरादयो वैयशः, यशोहेतुत्वात् , तेषु हि सत्सु ख्यातिर्भवति ; तथा वीर्यंबलम् अस्मिञ्शरीरे ; न ह्युत्क्रान्तप्राणो यशस्वी बलवा-न्वा भवति ; तस्मात्प्राणा एव यशो वीर्यं चास्मिञ्शरीरे,तदेवं प्राणलक्षणं यशो वीर्यम् उदक्रामत् उत्क्रान्तवत् ।तदेवंयशोवीर्यभूतेषु प्राणेषूत्क्रान्तेषु, शरीरान्निष्क्रान्तेषु तच्छरीरंप्रजापतेः श्वयितुम् उच्छूनभावं गन्तुम् अध्रियत, अमे-ध्यं चाभवत् ; तस्य प्रजापतेः, शरीरान्निर्गतस्यापि, तस्मिन्नेवशरीरे मन आसीत्; यथा कस्यचित्प्रिये विषये दूरं गत-स्यापि मनो भवति, तद्वत् ॥
In plain words
The scripture says, 'He desired...' to explain the true meaning of the words 'horse' and 'horse-sacrifice.' His desire was: 'Let me sacrifice once more with a grander, more magnificent offering.' The word 'once more' refers to his intent to perform the rite in a future life. In that future life, Prajāpati did indeed perform the horse-sacrifice. Deeply identified with the essence of that rite, he projected himself at the beginning of the cosmic cycle. Having become the very embodiment of the ritual's action, its instruments, and its fruits, he desired: 'Let me sacrifice again with a grander offering.' Having set his heart on this monumental task, he grew weary, just as ordinary people do. He then practiced intense inner contemplation. The phrase 'of him who was weary and heated' should be understood as before. The text itself explains what is meant by 'glory and strength departed': the vital energies—the sight and other senses—are glory itself, for they are the source of all renown; as long as they are present, a person enjoys fame. Likewise, strength is the physical power within this body. Surely, when the life-breath has left, no one is called glorious or strong. Therefore, the vital energies alone are the glory and strength in this body, and it was this glory and strength, in the form of the life-breaths, that departed. When these vital energies, which constitute glory and strength, had left the body, Prajāpati's physical frame began to swell and bloat, becoming impure and unfit for sacrifice. Yet, even though Prajāpati had departed from his body, his mind remained attached to it—just as a person who has traveled far away still dwells in thought upon the object of their love.
Close to the Sanskrit
The text says 'He desired...' for the purpose of explaining the etymology of the horse (aśva) and the horse-sacrifice (aśvamedha). 'May I sacrifice again with a greater, great sacrifice'; the word 'again' (bhūyaḥ) is used with reference to performing in another birth; that Prajāpati sacrificed with the horse-sacrifice in another birth; he, indeed identified with that state, manifested at the beginning of the cycle; he, having been accomplished as the very self of the action, agent, and fruit of the horse-sacrifice, desired—'May I sacrifice again with a greater sacrifice.' Having thus desired a great task, he became fatigued like an ordinary person; he performed penance; 'of him who was fatigued and heated' is to be understood as before; the text itself explains the meaning of the words 'fame and vigor departed'—the vital breaths, namely the eyes etc., are indeed fame, because they are the cause of fame, for when they are present, renown occurs; likewise vigor is strength in this body; for indeed one whose vital breath has departed does not become famous or strong; therefore the vital breaths alone are fame and vigor in this body, and thus the fame and vigor characterized by the vital breaths departed. Thus, when the vital breaths, which are fame and vigor, had departed and gone out from the body, that body of Prajāpati began to swell, to go to a bloated state, and became unfit for sacrifice; of that Prajāpati, although he had departed from the body, his mind remained in that very body; just as the mind of someone who has gone far away remains in a beloved object, like that.