The Brahman priest performs one of them in his mind, 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,

In plain words: One priest, called the Brahman priest, does his part silently in his mind. The other priests do their part out loud. This verse begins to describe what happens if the silent priest speaks during a certain stretch of the morning chants.