I'd say it's the connection with the world through your body. People say meditation is looking inside you, I start to think it's what they mean. You allocate your neurons to sense more about your own body and your interface with the world. Either the movement of weights and their effect on your muscles and joints. Or the vibration caused by your bow on the strings.

When I was working out, since I was out of shape, I used the music "slowness" as a guide to doing push ups. Always going slow, very slow, and very smooth.

Even on every day chores, I realize that anything done in this fashion is relaxing, body and mind wise.