Perhaps it's in putting the brain in a place where the body is on autopilot and the verbal / spatial centers of the brain can be free to wander in and out of focus.

Podcasts kill this mode for me, so you can't have the brain thinking about other people's thoughts or words.

Playing a video game doesn't work, so I think your spatial thinking has to be free too.

It's as if distracting the brain's non-verbal/spatial modes or burning some amount of calories from those centers gives the "thinking" parts of the brain some "alone time". That's an incredibly non-scientific and unserious hypothesis though.