Bhashya
अथ खलु इति प्रकारान्तरेणोपासनमुद्गीथस्योच्यते, व्यानमेव वक्ष्यमाणलक्षण प्राणस्यैव वृत्तिविशेषम् उद्गीथम् उपासीत ।अधुना तस्य तत्त्व निरूप्यते—यद्वै पुरुष प्राणिति मुखनासिकाभ्या वायु बहिर्नि सारयति, स प्रा- णाख्यो वायोर्वृत्तिविशेष , यदपानिति अपश्वसिति ताभ्या मेवान्तराकर्षति वायुम्, स अपान अपानाख्या वृत्ति ।तत किमिति, उच्यते—अथ य उक्तलक्षणयो प्राणापा- नयो सधि तयोरन्तरा वृत्तिविशेष , स व्यान , य साख्यादिशास्त्रप्रसिद्ध , श्रुत्या विशेषनिरूपणात्—नासौ व्यान इत्यभिप्राय ।कस्मात्पुन प्राणापानौ हित्वा महता आया सेन व्यानस्यैवोपासनमुच्यते ? वीर्यवत्कर्महेतुत्वात् ।कथ वीर्यवत्कर्महेतुत्वमिति, आह—य व्यान सा वाक्, व्या नकार्यत्वाद्वाच ।यस्माद्व्याननिर्वर्त्या वाक्, तस्मात् अप्राण- न्नपानन् प्राणापानव्यापारावकुर्वन् वाचमभिव्याहरति उ च्चारयति लोक ॥
In plain words
The meditation on the Udgītha is now approached through another method: focusing specifically on Vyāna, a particular modification of Prāna. When a person expels breath through the mouth and nose, that is the function known as Prāna. When they draw air inward through the same organs, that is the function known as Apāna. The intermediate junction between these two is Vyāna. Why is Vyāna singled out for meditation over Prāna and Apāna? Because it is the source of potent action—speech itself is produced by Vyāna. Thus, people in the world can utter speech even without exerting themselves in the acts of Prāna and Apāna.
Close to the Sanskrit
Now, 'atha khalu' indicates that meditation on the Udgitha is stated through another method, focusing on Vyana alone as a particular modification of Prana. Now its reality is ascertained: when indeed a person breathes forth through mouth and nose and expels wind, that is the breath-function named Prana; when he breathes down and draws wind inside by those same organs, that is the function named Apana. What then? Now, that which is the junction of those two described Prana and Apana, the intermediate function, that is Vyana... Therefore, because speech is accomplished by Vyana, one utters speech in the world without performing the activities of Prana and Apana.