When I am designing the solution, I still have the inner voice. But when I'm implementing in code, I can do it almost "brain-lessly", even fine when listening to music.

I'm not sure how do I do that, but I guess because I do it a lot, so I can do it as familiar as walking?

Edit: Maybe because I didn't learn programming by listening to teacher, instead I learn from books and trials. I guess it's more like walking / playing sport, you don't need explicit inner voice to guide your body movement.