I am quite sure from the context that the elbow of the spearing arm - which is about to catch and pull - is meant here. But I have to admit that I don't really get the point. When your arm is already in a forward stretched position how could you move your elbow forward ???
I have no clue what it means in the Shinji picture to move your elbow forward in the acceleration phase. The only thing that comes to my mind is not to drop the elbow in the catch phase - the elbow got to move backward anyway sooner or later... particularly in the acceleration phase.
Amazing that people can get the breakthrough of their life out of this...