In the movement " straight ahead", the arm leaves in the axis. With my poor English, it's difficult to explain better.
An illustration of a "trebuchet" style. (Old Indian)
http://www.youtube.com/user/ruebenschulz#p/a/u/0/Dqtv2o3T2AIFrom the position of the hand "palm to top", the Greek drawing would plead for "trebuchet". However the drawing represents the starting position but il does not describe the evolution of the movement.
Just my conclusions and I can be wrong. But I love this work.
Looking for old gestures is a very random undertaking. And for those who are interested, the position of the feet on Makron cup (shared by Thearos) is identical but in a war context.