The shoulder girdle and pelvic girdle are even - there is a snapping of the pelvis and a thrust forward of the chest when the body is held straight.
Alan wrote an article of a basic training exercise I teach with the medicine ball. You can see the ilustration in the article:
http://www.alanorr.com/htdocs/articl...erarticle.html
Better yet, try it with a 15 or 20 lb medicine ball. Try it hunchbacked, then try it with straight body and report back to us.
Hunchback training is fine if you want to look like Quasimodo. It messes up the alignment of the internal organs, is poor for the spine, and bad for your breathing, and health. You can talk to orthopedic doctors, chiropractors or acupuncturists about this. In yogic and qi gong traditions poor posture is also incorrectly misaligning the chakras, calling for disharmony between the emotions, and in mental and spritual development.
I am not sure if Sum Nung taught it incorrectly, or whether people interpreted it wrong. I am also of YKS lineage, and have great respect for the lineage, but one has to look at YJKYM from a point of view of health, alignment and martial context.
WCK should not make you look like a gnome. :) If Wing Chun or Ng Mui did make an art like WCK, would they want to look like a dude wasted on opium? Perhaps that is where the posture comes from.