I don't know about that. I saw a recent pic here of a younger rider with the head so far up I expect it to snap off and the back rounded to the extent that with the leg there is a very pronounced "C" shape albeit a backwards one.
Fluidity, whether slightly convex, concave or flat. No extremes.
I think jaydee hit on something I missed being focused on the leg. Shoulders should follow and not be pinched back.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.