According to George Morris:
"The trunk of the body should be carried erect with the shoulders relaxed and the chest opened. A slight arch should support the upper back, but care must be taken not to arch the lower back, as this will bring the seat out of the saddle... Two commonly observed posture faults are the stiff, hollow, "swayback" and the sloppier, hunch-shouldered, rounded "roachback". Both of these undermine the use of weight as an aid.
I think the key words are SLIGHT ARCH otherwise you get into the stiff "swayback" catagory