Verse 8.5.2

अथ यत्सत्त्रायणमित्याचक्षते ब्रह्मचर्यमेव तद्ब्रह्मचर्येण ह्येव सत आत्मनस्त्राणं विन्दतेऽथ यन्मौनमित्याचक्षते ब्रह्मचर्यमेव तब्ब्रह्मचर्येण ह्येवात्मानमनुविद्य मनुते ऽ॥ ८.५.२ ॥

atha yatsattrāyaṇamityācakṣate brahmacaryameva tadbrahmacaryeṇa hyeva sata ātmanastrāṇaṃ vindate'tha yanmaunamityācakṣate brahmacaryameva tabbrahmacaryeṇa hyevātmānamanuvidya manute || 8.5.2 ||

2. Then, that which is known as ‘Sattrāyaṇa’ [a sacrifice lasting a long time] is brahmacarya, for it is through brahmacarya that the individual self gets liberated [attains union with the Cosmic Self]. Then, that which is called ‘mauna’ [silence] is brahmacarya, for through brahmacarya one realizes the Self, and having realized the Self one remains absorbed in the thought of it.

Word-for-word explanation:

Atha, then; yat sattrāyaṇam iti ācakṣate, that which is called a ‘Sattrāyaṇa’ [a long sacrifice]; tat brahmacaryam eva, that is brahmacarya; hi, for; brahmacaryeṇa eva, through brahmacarya; sataḥ, from Sat [i.e., the Cosmic Self]; ātmanaḥ trāṇam vindate, the individual self attains its liberation [union with the Cosmic Self]; atha, then; yat maunam iti ācakṣate, that which is called ‘mauna’ [silence]; tat brahmacaryam eva, that is brahmacarya; hi, for; ātmānam anuvidya, knowing the Self; brahmacaryeṇa eva, through brahmacarya; manute, one remains absorbed in thinking.

Commentary:

There is a particular sacrifice called Sattrāyaṇa, which involves the services of many priests. It is a big affair. The Upaniṣad says it is considered brahmacarya to perform this Sattrāyaṇa sacrifice because through Existence (sat) a person gets protection (trāṇa) for oneself (ātmana). You always feel you are protected by Sat, so you are sure of yourself. In the same way, when you are practising brahmacarya, you know nothing is going to sway you from the vow you have taken.

As the Gītā says, the self is your own friend and it is also your own enemy. By practising brahmacarya you will gradually become stronger, and as you grow

Mauna, a vow of silence, is also a form of brahmacarya. We all know that when we are doing something serious we like to be quiet. We don’t like to talk much, nor do we like our mind to be restless. We want to be able to fix our mind on what we are doing. Similarly, brahmacarya means that our mind is fixed on Brahman. We are in touch with Brahman, always reminding ourselves that we are not this body and not this mind. We are the Self. We are Brahman. So taking a vow of silence (mauna) means that a person contemplates, or meditates, (manute) on his identity with Brahman.

Śaṅkara says that this meditation on Brahman comes after seeking the help of the scriptures and the teacher. Both are necessary. The scriptures give you guidelines, but the teacher takes you by the hand and leads you to the goal.