Some Wing Chun doesn’t have the sunken chest and raise back. The curl is what my sifu calls it…
In wing chun the correct body structure is as follows the kuit kuen
Yee Jee Kim Yeung Ma
1.Pull in the chest, push out the upper back, and bring in the tail bone.
2.Fill the Tan Tien with chi and distribute the strength to all parts of the body.
3.Point the knees and toes inward.
4.Form a pyramid with the center of gravity in the center.
5.Fists are placed by the side of the ribs but not touching the body.
6.Sink the elbows, the shoulders, and the waist.
7.Hold the head and neck straight and keep the spirit alert.
8.Eyes are level, looking straight ahead, and watching all directions
This is similiar to Tai Chi Chaun
There should be a slight drawing in of the chest which allows the chi to sink to the “Dan Tian”. Avoid protruding the chest as this will cause the chi to rise which will lead to top heaviness, and the soles of the feet to float. Raising the back means that the chi adheres to the back. If you can sink your chest, your back will naturally rise. If you can raise your back, your power will come from your spine enabling you to overcome any opponent. Thus the Chi can sink to the Dan-Tien, and also raise in the back. If this principle is not understood correct, it will not be possible to obtain Geng (rootedness) in the feet, and the whole body will be unstable. Sink the chest and raise the back are similar to when a cat is in readiness to launch an attack on its prey.
http://www.powerattunements.com/taichitheory.html
http://www.wingchunpedia.org/pmwiki/pmwiki.php?n=WCP.KuenKuit#Yee
PICTURES OF SUNKEN CHEST RAISED BACKS!!!






RELAXING YOUR CHEST, SHOULDERS AND WAIST WILL CAUSE YOUR CHEST TO SINK ESPECIALLY IF YOU TUCK IN YOUR PELVIC!
http://www.mainewingchunkungfu.com/article15.htm
http://wingchunipman.com/Tradition_KuenKuitOfFormsStance.htm