As a sometimes runner and jazz sax player (intermediate), I can completely relate to this thread on both topics. One thing I noticed when studying a new improv technique or parsing out chords and applying great mental strain, a few days / weeks after said activity, I will dream that I am playing my sax and actually do a pretty nice little improv in my dreams. When I wake up, I can barely remember the melody. This has happened about a half dozen times. My reasoning is that in my waking moments I over think things, e.g. What is the right line to play over this or that progression. I guess in my sleep, I am released from all these heuristics and am able to tap into a deeper, freer layer. A jazz teacher once told me to just forget about the theory - not overthink stuff - and just play what I feel. It sounds cliched but it's good advice. (He was a mathematician by training so could relate ).

This is why I didn't invest in theory, I wanted to feel the music. It happens once in a while and I can surf the music's surface for a minute totally free. It's a bit crazy but in these moments the brain really sees things like smooth hills and if you're tuned in, you're like a ball rolling in these, picking a bit where you'd like to roll next, or maybe bounce because why not.

There's a thing you get after a while on drums, the internal clock becomes stable. You can play whatever, if you feel you don't make sense, a part of your mind is still ready to catch the groove as if you never played outside the click.

In Harmony there's a similar sensation, you don't care about the note, any note on any chord will taste something, but you learn to swerve back into the harmonic movement if you think you're turning into noise, or if you want to express something sweeter to the ear, depending on your emotions and the overall flow.

I believe to learn that, the best way is to play off. Off time, off key. And feel what it is to lock in again. Then play on that sensation.

I play mostly bass dont worry. I think the explanation is odd but easy. I tried singing guitar solo (little wing etc). Tried to expand range, and when it was above, I started to try to sing other instruments or backing vocals. Started with unison and fifth, then 9th etc etc. I also listened to contemporary gospel and acapella (take 6, brothers voices only, sometime they imitate instruments too, Google take 6 - jazz vive).

Also jazzfusion guitar chord progression got me to focus on funny progression. John scofield, tribal tech, r. Gatto, return to forever...) All reach your ears.