100/0, 70/30, 60/40, 50/50. There are several different theories as to which is the proper weight distribution of the stance in Wing Chun Kung Fu.
In Wing Chun Kung Fu different systems believe in using different weight distribution in their stances. This discussion will not be talking about the origin of the different weight distribution, but rather looking at the differences, positives and negatives of the different weight distribution of the different stances.
100/0, 70/30, 60/40
In these stances when shifting from the neutral stance 100%, 70%, or 60% of the practitioners bodyweight is shifted on to one leg or the other. The lighter leg would then have the other 0%, 30%, or 40% of the practitioner's bodyweight. The thinking behind this is, the front leg is lighter than the rear leg. Due to this lightness the practitioner supposedly would not be able to be swept on that front leg. There is also the belief the practitioner would then be able to kick more easily. Now this part is true. It is easier to kick when all of or part of your weight is on one leg. Unfortunately, this will hinder the mobility of the practitioner. If all or part of a person's bodyweight is on one leg, how can that person move in all directions equally? They won't be able to. Another disadvantage of an unequal stance has to do with the structure of the stance. If a person has unequal body they are not as structurally sound as a person with equal body weight. Which is easier, standing on one leg or two legs?  This is not rocket science in why the structure is weak.  If you take something as simple as a top and spin it, at its peak and when its fully spinning, the center of the top is straight and doesn’t move.  If you hit the top at this point, the top continues to spin and maintain the center while adjusting to the force of the hit.  Now take that same top and if that center is not shifting from side to side, any contact made upon it will immediately knock it down.  If Wing Chun is based on structure then you definitely want to maintain the center in your stance and not give it up.
The reality is, that you will do what comes naturally to you.  Its true that you train to develop, but when it comes to natural motion, the human body will do what’s natural.  Regardless of whether the Martial Arts is Wing Chun, Tai Chi, Wu Chu, or Shaolin, to name a few that can have awkward stances, you will move how your ingrained to move when it comes to a fight.  Think of all the Martial Arts that I mentioned, and the different stances that they can take, have you ever seen once, just once, any of those stances actually applied in a fight.  The answer is no, because, one in a real fight situation you can’t pose to fight, and two your gonna move the way you were meant to move . The answer is obvious.
In the 50/50 stance the weight of the body is equally distributed to each leg. When shifting, each leg will always have 50% of the weight on it. This allows for the two themes behind the 50/50 stance. One, it allows for better mobility and two, it allows for better foundation and structure. The mobility part is better due to the fact there is no need to continuously transfer weight from one leg to another. Using the 50/50 stance one would move as easily and naturally as walking or running.   In fight situation you want the feet to be used for what it does best and that’s move.  No other stance other then 50/50 can give you better mobility and a better chance at winning in a fight.  When a sprinter runs is the person keeping more weight on one leg or the other? When any person is walking are they keeping more weight on one leg than the other? The answer is no. As for the structure aspect of the 50/50 stance, let me give you this example. When riding a bicycle is it better to ride on one wheel or two? People may think, well what if someone tries to sweep me, shouldn't I have a light leg? No, having one leg lighter than the other isn't going to save you. Your skill as a martial artist is going to save you.  When it comes time to close the gap, nothing can stop a well timed and structured rush attack.  
The common theme in all of Wing Chun is to do what's natural. Everything, and I mean everything you do in Wing Chun is to do what comes naturally for your body and coz you can fool the body to do things when in class, however when the heat is on, your body will react appropriately.

Written by Sifu Ken Weingart
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