When I played guitar ,my wife used to ridicule me unmercifully for My "jam face" I sometimes think it is mostly related to guitar players. Your kind of one with the guitar bending the strings and requiring some touch and effort to achieve sounds. Since I'm a keyboard player ,I seem to just smile all the time , I really don't have a jam face on the keys. I think she used to get embarrased pretty bad about it.

I don't have a face thing but I have a neck thing. I bob my head to the beat and twist (kind of like body english) to accentuate certain parts. Not that I do this conciously, I noticed it watching films of the band.
I used to work with a guitar player who would mouth out all of his leads. It was hillarious to watch him play blues when he did his long drawn out bends.

Never played a gig, and not even in a band, but when I jam on the bass my wife says if I keep making that face she is going to take it away from me.

I am told I look like I am in pain when I play. I usually reply, "I am."