It's easier to explain through an example.

Your voice gets quieter. T mirrors you and quiets his own voice as well.

One time, a previous therapist wore the same zip-up I was wearing, but in a different colour. I joked that he didn't have work that hard at mirroring me in session.

My current T does it as well, though he is so smooth about it, I don't notice it right away. He might turn on his compassionate voice if I'm talking about something difficult or if I'm upset. This change makes me feel like he is really paying attention and is attuned to what is happening during the session.