Originally Posted by
timtak
Sir Bradley has his weight on his arms with his back arching down..
That's actually arching up and that's the correct posture (in aero TT posture)!
It greatly contrasts OP's back posture. It seems we have a miscommunication of what arching up or down is. For more upright riding position, the less you need to arch your back up.
The part most critical is the lower back and that is where it needs to be arching up.
The back arched up posture is mainly for unloading the core muscles and the arms, great for long rides. In a lesser extent, it improves aero but the gains are mostly on comfort and avoiding lower back injury this even more important if you're actively recruiting your glutes to pedal. I read it on your previous post.