"Flow". I have also heard it called "fugue".

When you have mastered an activity so totally, so completely, that you no longer expend any mental effort at carrying it out, and you can use it as a medium to carry out some other goal or intention.

It can be riding a horse, driving a sports car, playing an instrument, or boxing a worthy opponent. You can't write good poetry until you have first mastered the language; vocabulary, grammar and syntax, of the language the poem is written in. If you have to think about what you're saying, and how, you will be distracted from what you want to say.

Maurits Escher, the Dutch artist, described it (and I paraphrase)
as

"Mastering one's craft so totally, so completely, that one need no longer give it any thought; leaving the artist free to use his art and his craft directly, so that his soul is free to work unencumbered and free of effort."