Avatarika
रहस्यप्रकरणे प्रसङ्गात् आरण्यकत्वसामान्याच्च यज्ञे क्षत उत्पन्ने व्याहृतयः प्रायश्चित्तार्था विधातव्याः, तदभिज्ञस्य च ऋत्विजो ब्रह्मणो मौनमित्यत इदमारभ्यते—
In plain words
In the esoteric section, following the context and the shared forest-text tradition, when a flaw occurs in a sacrifice, the mystical utterances are prescribed as expiation, alongside the silence of the priest who knows this truth. Hence this text begins.
Close to the Sanskrit
In the esoteric section, due to context and the commonality of being forest texts, when a flaw has arisen in a sacrifice, the vyāhṛtis are to be prescribed as having the purpose of expiation, and the silence of the officiating priest who knows that—therefore this is commenced.