Verse 4.16.2-3

तयोरन्यतरां मनसा संस्करोति ब्रह्मा वाचा होताध्वर्युरुद्गातान्यतरांस यत्रौपाकृते प्रातरनुवाके पुरा परिधानीयाया ब्रह्मा व्यवदति ॥ ४.१६.२ ॥
अन्यतरामेव वर्तनीं संस्करोति हीयतेऽन्यतरा स यथैकपाद्व्रजन्रथो वैकेन चक्रेण वर्तमानो रिष्यत्येवमस्य यज्ञोरिष्यति यज्ञं रिष्यन्तं यजमानोऽनुरिष्यति स इष्ट्वा पापीयान्भवति ॥ ४.१६.३ ॥

tayoranyatarāṃ manasā saṃskaroti brahmā vācā hotādhvaryurudgātānyatarāṃsa yatraupākṛte prātaranuvāke purā paridhānīyāyā brahmā vyavadati || 4.16.2 ||
anyatarāmeva vartanīṃ saṃskaroti hīyate'nyatarā sa yathaikapādvrajanratho vaikena cakreṇa vartamāno riṣyatyevamasya yajñoriṣyati yajñaṃ riṣyantaṃ yajamāno'nuriṣyati sa iṣṭvā pāpīyānbhavati || 4.16.3 ||

2-3. The priest called brahmā in a sacrifice purifies one of these two paths [i.e., the path of the mind] by his [discriminating] mind. The hotā, the adhvaryu, and the udgātā priests purify the other [i.e., the path of speech] by [chaste and elegant] speech. If, however, the brahmā priest breaks his silence when the morning anuvāka has begun, before the paridhānīya Ṛk hymn has been read, then only one path [the path of speech] has been purified. The other is ruined. Just as a one-legged person trying to walk, or a one-wheeled chariot trying to move, is doomed, in the same way the sacrifice is ruined. And when the sacrifice is ruined, the sacrificer is also ruined. In fact, the sacrificer is even liable for having committed a sin by performing the sacrifice in that way.

Word-for-word explanation:

Brahmā, the priest called brahmā [in a sacrifice]; tayoḥ anyatarām, one of these two; manasā, mentally; saṃskaroti, purifies; vācā, by speech; hotā adhvaryuḥ udgātā anyatarām, the hotā, the adhvaryu, and the udgātā priests [purify] the other; yatra, when; prātaḥ anuvāke upākṛte, the anuvāka which is read in the morning has begun; purā paridhānīyāyāḥ, before the Ṛk hymn called paridhānīya; saḥ, he [the brahmā priest]; vyavavadati, breaks his silence; anyatarām eva vartanīm saṃskaroti, only one path [i.e., the path of speech] he purifies; anyatarā, the other [the path of the mind]; hīyate, is spoiled; yathā, just as; ekapāt, a person with one leg; vrajan, walking; vā, or; rathaḥ ekena cakreṇa vartamānaḥ, a chariot moving on one wheel; riṣyati, is doomed; evam, likewise; asya yajñaḥ riṣyati, his sacrifice is ruined; yajñam riṣyantam, when the sacrifice is ruined; yajamānaḥ anu-riṣyati, the sacrificer is also ruined; saḥ, he [the sacrificed; iṣṭvā, having performed the sacrifice in this way; pāpīyān bhavati, becomes a sinner.

Commentary:

In the previous verse, two paths were mentioned for the performance of a sacrifice—the path of speech (vāk) and the path of the mind (manas).

In a sacrifice, there are four types of priests: brahmā, hotā, adhvaryu, and udgātā. The brahmā priest is supposed to purify the path of the mind by his own purified mind, while observing silence. The other three priests take care of the path of speech, purifying it by their pure words.

But suppose the brahmā priest breaks his silence while the reading of the morning anuvāka is going on, before the paridhānīya Ṛk hymn has begun. He is supposed to remain silent then, with his mind on a high level. Through this he is to purify the path of the mind. But if he breaks his silence, his mind is no longer ‘pure.’ Then the sacrificer has to make do with only one path—the path of speech. His position is now like that of a one-legged man trying to walk, or like a one-wheeled chariot trying to move. Both the sacrificer and the sacrifice are doomed, and the sacrificer is to be regarded as having committed a sin.