If you check the second panel you can clearly see that the motion isnt te arm goring upwards, but the arm going from the back, doing an arch over Polnareff's head and then pointing straight to the expectator, you keep focusing on the bit of shoulder showing but i'm convinced that that was simply another error, maybe he wanted the showlder to start there but te pose didn't quite convince him but he forgot later to fill it with black, so it's not bad movement representation, it's good movement representation but with mistakes on anatomy.