Ok,
Out here in Guangzhou, wing chun has 2 fighting platforms. The first we learn is called Lei Kyu Chi Da (it means fighting without a bridge) This has to be learnt before you ever touch Chi Sau. Basically it is fighting from a distance, learning to close the distance through correct stepping techniques. Most importantly it is IMHO learning to fight a "real" Fight using Wing Chun. In all reality for the most part only a few moves are needed from Wing Chun to fight at a distance, Daap, Gou Laan, Laap Ngoi Lim, Ji Mh Choih, and Fu mei being what I would think of as the bare essentials. Guangzhou's wing chun covers this with its Sahp Yih Sik and San Sik fighting, before ever engaging in Chi Sau.
In the beginning this is choppy with no real sticking, no real controlling of the opponents balance point (this is due to not learning chisau, not a fault in the system itself) what you end up learning to do is using wing chun stepping methods to out manover and out fight other fighters. (in theory. it really depends on how hard someone trains and HOW they train to whether they can make it work or not)
after this has been learnt and trained. you move into Chi Sau through some basic exercises, Dan Chi and then Seung Chi, In Chi Sau you learn the sticking, controlling, and shutting off of your opponents limbs, normally it comes in the form of rolling hands. Why do we roll hands? It really doesn't apply to fighting from my experience, it is IMHO just a platform to come off, a little less likely to hurt someone if you are already touching and rolling bridges than if you come in from a distance ( in the Chi Sau comps I have seen out here, the only time they Roll is when they are training, or having a friendly play, when actual comps have gone down, their hands meet and it's on). It is NOT necessary for fighting, If your Lei Kyu Chi Da is good, you do not have to rely on sticking to win a fight. Although Everything you find in Chi Sau is found in Lei Kyu Chi Da. The difference is in Chi Sau, you learn to apply the techniques while someone is adding pressure to,/trying to control your arms, position, and balance point. This way of fighting we call Chi Kyu Chi Da. (fighting with a bridge)
Now for all intense and purposes fighting with a bridge is a lot easier than without.
Why?
If I am standing at a distance from a person, then he has 2 arms, 2 legs and a multitude ofways he can attack you. Thats a lot to think about.
If I have a bridge through feeling their balance point, you have a better knowlege of what they can and might do from the way they reat to your bridge. You are simply cutting down the number of choises your opponent has as giving you less to worry about.
These two do mix. Once you have learnt Chi Sau, then you learn/are able to go from a distance, control the distance, get up closer which is slightly preferable for wing chun, once your close what you have learnt from Chi Sau. the feelings, the reactions will come into play. It is doubtful you will actually chisau for any legnth of time, more you close the distance obtain a bridge, control/nutralise the bridge, smash the crap out of the guy and the fight will be finished.
I have found though, much to my dismay a fair few Wing chun practitioners ignore the Lei Kyu Chi Da part of training to focus on the more fun, Chi Sau.
Just my 2 pence worth.
WJL