It’s his other hand. There is a reason we only see one half of his body. You can see that the hand enters from the opposite direction. And the hand motion is not a perfect replication. You can also see his body moving while his left hand is moving.
It's just a trick with an elliptical mirror seen from another angle. It really is the other arm that he moves. I didn't expect this from this great artist, but maybe that's the provocation. In the Netflix documentary (Abstract, episode 1) he explains it better. It wasn't me who realized the trick, but my teenage students during a class!!!!
