Yes, you're right, his core is doing a lot of the work.

I meant no muscular effort in his arms. Despite his core strength, his arms are supporting some of his weight, and he has them just slightly flexed. Compared to this old style racer, with no saddle to bar drop, who has a flat back but with his elbows deeply bent.



Looking at this picture helps me understand where some of the old terms came from. "Gnawing the stem" and "On the rivet".