अथ योऽस्योर्ध्वः सुषिः स उदानः स वायुः स आकाशस्तदेतदोजश्च महश्चेत्युपासीतौजस्वी महस्वान्भवति य एवं वेद ॥ ३.१३.५ ॥
In plain words
Its upper opening is the up-breath, and that is the wind, and that is space. Let a man worship it as vigour and greatness. He who knows this becomes vigorous and great.
The commentators
Chāndogyopaniṣadbhāṣyam· bhāṣya
अथ योऽस्योर्ध्व सुषि स उदान आ पादत लादार भ्योर्ध्वमुत्क्रमणात् उत्कर्षार्थ च कम कुवन् अनितीत्युदान । स वायु तदाधारश्च आकाश । तदेतत् वाय्वाकाशयोरोजो- हेतुत्वादोज बल महत्त्वाच्च मह इति । समानमन्यत् ॥
In plain words
Now, that upward cranial passage is udana. Spanning from the foot all the way to final departure, performing actions while breathing upward, it is udana itself. That breath is air, and its resting ground is space. Because it is the source of vital vigor, and because of its immense strength and vastness, it is designated as 'maha'. The rest follows the same pattern.
Close to the Sanskrit
Now that upper aperture of this [body], that is udana, beginning from the foot and upward to departure, performing action and breathing thus—udana. That is air, and its support is space. That this is called 'maha' is because of being the cause of vigor, and because of vigor, strength, and greatness. The rest is similar.