I've been exposed to many different families of Yip Man's varied versions since the early 90's and I've found this basic system(s) to exemplify a softness or yielding of force, utilizing opponents energy etc., basically the same principles just varied application or interpretation depending on that particular lineage or school. Yip Man Wing Chun has a very scientific/geometrical approach to center line theory, simultaenious attack/defense while either playing off the opponents energy or mearly helping that energy along it's path, while delivering one's own defense/attack. Although I've seen it written, suggested, hinted at that one can perform SLT ever so slowly and practice it in a "Qiqong" mindset, it for the most part is not practiced for that purpose. The practice of Chi Sao and veried other drills does target sensitivity and proper allignment of the different techniques/positions thus creating the wedge/angle/cutting attributes so often found in Yip Man systems today.
Pan Nam's W.C. does take a bit of a different approach being a bit looser in it's centerline theory by Y.M. standards, but once one understands the principles behind it, it truly does make sense especially after you begin the Pan Nam Chi Sao practice. Pan Nam's from the very beginning (SLT) develops the flow of Qi and utilizing "breath" energy, yes still keeping the simultaenious attack/defense principle active but with an explosive delivery of Qi/soft energy force at the point of contact. Pan Nam's utilizes a more attack from an angle, rather than creating the angle yourself and has a much more brutal delivery using veried techniques a bit different from the standard Y.M. practice. In my opinion, and that's all it is - my opinion, I feel that knowing these two versions, seemingly different, yet giving a more total/complete picture of Wing Chun and perhaps being able to see the possible evolution of this art throughout it's history. Pan Nams utilizes even more theory of "flowing with the opponent" than Yip Man's, yet Yip Man's has a more direct approach etc. I feel that Wing Chun is a training system, not necessarily a fighting system, and as training progresses so does your own personal Kung Fu, as long of course as you keep within the theories, principles and precepts of Wing Chun. Knowing both has truly brought about a ballance to my own Wing Chun, and I can flow from one attribute to the other with ease, as I feel they complement each other's energy and are able to be used together if you will, flowing within each other....after all, they're both Wing Chun.
Humblest Respects,
BFL