As a computer programmer and a musician, I have experienced is crossing over from "still learning to do it" into "performing with little effort". I assume anyone who drives a car or rides a bike has the same experience.
My experience is similar but a tad different.
There is perhaps just one song that I can play along to on the bass where a) I have it so memorized and that I have such "relaxed conscious control or supervision" that it's as if I'm on some kind of auto-pilot and b) where I experience a state of near-euphoria. This song is
Song on the Radio by Al Stewart. Others may call this "being in the zone". It's more than just locking in with the drums, it's like riding a wave of vocals, keyboard and sax with your fingers knowing you won't fall off the surf board. Maybe another way to express this is being part of a "larger truth"; that is, the band. I am able to improvise on this song but only a little bit; my bass skills hold me back from fully improvising
without error. The funny thing is that I have yet to reach this "zone" with any of the stuff I've actually composed and recorded.
Likewise, over my career there were times where I'd be in a similar "zone" while writing code or working a tough engineering problem late at night when only the janitors were around. But I can't say that I reach a similar zone while riding a bike, driving a car, or speaking/writing English. Perhaps, the "danger" of messing up or a real sense of accomplishment needs to be present. Or maybe joy has something to do with it. Riding a bike or speaking is so easy there's no danger, joy or sense of accomplishment involved.
Perhaps this isn't about
intensive hyperfocus on a task or relaxing one’s focus but rather some combination of the two. In any event, this is a fascinating subject.