As sifu has taught me, we never, ever move back, which does not mean to say that we do not employ Foot work, Sifu also told me that if I thought I could just stand there I was stupid (he is actually a nice man

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The reasoning behind not going backward are two fold:
A) as has already been stated, correctly, Wing Chun is a forward pressure art, if you cannot maintian that forward pressure, then yu have not learnt the skills of Wing Chun. You will notice with the rare clips of WSL that there is no backward from him, but a continual forward pressure, penetrating the opponant, and striking deep beyond his confined area. Here the principle of Loy Lau Hoy Sung comes into play
Loy
Come Lau Receive Hoy Go Sung FollowReceive what comes send of or (follow) what retreats. Doesn't have the meaning that most people attach to it. The Essence of this concept is in the utilisation fo the confined area, together with th Sui Lum Tao energy, so that you are controling the game, and this principle utilised with these aspects will mean that yu will never, ever ahve to step back, but if you do not understand these fundamental ideas, concepts or have not developed the Sui Lum Tao energy, then you will not have achieved anything, and you will not be able to make it work.
2) The second is that going back will cause a wrong concept in the execution of the Wing Chun foot work (which is why so many people think that the foot work is too static and stiff [unworkable]). The ide3al of the foot work is outlined in the Chum Kui form, the fundamental problem that a Wing Chun practicioner has is "hw do I move and maintin my Sui Lum Tao energy?", if you you always see the Wing Chun footwork in terms of direction rather than position, then you will find yourself becoming lost. There is no backwards in Wing Chun, just as there is not forwards, and no sideways, there is simply an optimal position to be in, now you may say well thats the same as moving forwards, sideways etc. in Some regards that is true, however to simply say you are moving forwards is a fallacy to some degree, in a fight I might be proud of the fact that I have moved forwards, but if I cop a right hook in the temple, my boasting days are over, howevwer if I adopt the way of thinking of position rather than direction, i will find myself looking for more meaningful ways of transporting my Sui Lum Tao energy to THEM. I dont want to run, I want to utilise my SLT energy which can only be done from the perspectinve of "ultimate position"