Ahh the arm thing. That’s a false bout. Many electric violins have something like it. It is used as a reference point for playing in higher positions. Without it, many advanced violinists can’t play very precisely in those higher positions and they complain about it a LOT.
I never go up that high, so for me it’s a nice handle, but I do not like the aesthetic of it.
Reminds me I need to make a V2 neck that doesn’t have the false bout.
On the hardanger fiddle there is something similar, but that’s used to support the base of your hand when playing in first position. Most players hold the instrument with their wrist bend resting their hand on the bout of the instrument. It’s a little strange.
-David