Tanya Chapter 35 part 5

Yes, the animal soul at its core remains the same even during a mitzvah. But because its power is subjugated to holiness during a mitzvah –– indeed! It even powers the mitzvah –– and its essential self is rendered voiceless although unchanged, it does not prevent the Divine presence from manifesting upon the totality of the animal soul and upon the entire body. The act of a mitzvah manifests the Divine presence to varying degrees. The Divine soul and its garments, along with the power of the animal soul that impels the mitzvah, are totally absorbed within the Divine light. And as a result, a ray of light is drawn over the totality of the animal soul and above the entire body in an encompassing way. But none of these differences contradict the ultimate truth –– that Hashem is One without any differentiation or multiplicity ch’v.