In fact we could find the waist working in improving the force of punching in Karate as well. This was no more a secret to many martial artist.
In order to use this technique, one needs to straighten the back of body and the spinal cord. There was a relief from too much pressure on our back and waist portion. Thus the upper body could spin left right more freely.
However the authentic style of Wing Chun would prefer to bend in the shoulder to the middle line of body, and to create a longer distance from elbow to the chest. There was much pressure on the back part of body. True, the waist was locked up more easily. However, there would be internal injuries at this part of the body in long run. I was suffering from the aching for many years after the muscles were put into stress time and again. I do not want to challenge the authenticity of the style, however having said that I would like to offer a different option for learner. I have put forward a revolutionary suggestion to the Wing Chun martial artist. And I have adopted this method and my training was improving very well.