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I found the English name for this "gymnast wing moving". This "gymnastic" is very usual for flies from Sepsidae, Opomyzidae, Pallopteridae as far as I observed. It is rather strange as all 3 are from different superfamilies. On the other hand I've never seen "gymnast" wing moving of Dryomyzidae or Sciomyzidae from the same superfamily as Sepsidae.
I didn't find yet any explanation of this neither in books nor here in Diptera.info