The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. Verily, he who purifies (Vâyu) is the sacrifice, for he (the air) moving along purifies everything.
[paragraph continues] Because moving along he purifies everything, therefore he is the sacrifice. Of that sacrifice there are two ways, by mind and by speech.
2. The Brahman priest performs one of them in his mind 1, the Hotri, Adhvaryu, and Udgâtri priests perform the other by words. When the Brahman priest, after the Prâtaranuvâka ceremony has begun, but before the recitation of the Paridhânîyâ hymn, has (to break his silence and) to speak,
3. He performs perfectly the one way only (that by words), but the other is injured. As a man walking on one foot, or a carriage going on one wheel, is injured, his sacrifice is injured, and with the injured sacrifice the sacrificer is injured; yes, having sacrificed, he becomes worse.
4. But when after the Prâtaranuvâka ceremony has begun, and before the recitation of the Paridhânîyâ hymn, the Brahman priest has not (to break his silence and) to speak, they perform both ways perfectly, and neither of them is injured.
5. As a man walking on two legs and a carriage going on two wheels gets on, so his sacrifice gets on, and with the successful sacrifice the sacrificer gets on; yes, having sacrificed, he becomes better.
68:3 If any mistakes happen during the performance of a sacrifice, as described before, they are remedied by certain interjectional p. 69 syllables (vyâhriti), the nature of which is next described. All this is supposed to take place in the forest.
69:1 While the other priests perform the sacrifice, the Brahman priest has to remain silent, following the whole sacrifice in his mind, and watching that no mistake be committed. If a mistake is committed, he has to correct it, and for that purpose certain corrective penances (prâyaskitta) are enjoined. The performance of the Brahman priest resembles the meditations of the sages in the forest, and therefore this chapter is here inserted.