It's a very real phenomenon, Concertina face...there must be cross connections in the brain, or summat like that. I see mostly lip movements among my fellow concertinists...but most of them play English while I play anglo. I try not to watch in order not to laugh out loud, though I know I also make faces while playing...can't be helped.
Another noticeable side effect is synchronous breathing...not so much holding of breath as per squeezergeezer...inhaling and exhaling matched with the motion of the bellows...which can lead to hypoxia or hyperventilation depending on the tune...
Anyone else notice that? It took me a conscious effort to de-link my breathing from my anglo-pumping.
