You smash your thumb with a hammer, you strum searing chord on an axe…you’re “getting busy”…and your face is screwed up and contorted.
What in your brain is “lit up” in common with the three activities?
Ask Robin Trower. He knows.
Robin Trower? Pbbfft! Go ask the Axis. He knows everything…
WAG: it probably has to do with trying to concentrate.
ask the Vapors…
‘Turning Japanese
I think I’m turning Japanese
I reallly think so’
So when a Japanese person hits their thumb, strums a bad chord, or gets busy, do their contorted faces look Caucasian? Do they call it ‘turning Occidental’? 
I’ve seen Trower onstage. During solos, his eyebrows were quite mobile, and his mouth seemed to be mostly saying, “Ooooo.”
B.B. King, on the other hand, runs through a variety of expressions while playing, including a wide open smile when he hits a really rich note.
My uncle and I were watching an old bluesman on television. His expression was a combination of emotional pain and musical pleasure. Sometimes he slowly shook his head from side to side, as if to say, “I don’t believe it myself.” My uncle asked me what the head-shaking was about, and I didn’t have an answer for him. 