Many people understand double weight in Tai Chi as distributing of equal weight on both feet. This is only partially correct. Double weight is more than that. The opposite of double weight is double float. I have explained in detail both of these deficiencies in my book - How To Master Tai Chi Chuan's Internal Skills. Suffice to say double weight is too much Yang and double float is too much Yin. We commit double weight fault only when we oppose force vs force. 50-50 weight distribution on feet is in itself OK. It is wrong only when we use 50-50 stance to withstand an oncoming force. Why? Because when the outside force lands on your body, according to Tai Chi principle you should neutralize it by playing with substantial and insubstantial, Yin Yang concept. When opponent is hard, you are soft and vice versa. Similarly when force lands on your body, that part of the body must empty out and become Yin. And corresponding to that, your Zhong Ding (vertical center) shift as well, this must be supported by distribution of weight in your feet. You cannot neutralize effectively with a 50-50 stance, in this stance you will inevitably fight force vs force to stay balance. Double weight also applies to hand, body and anypart that comes in contact with outside force, including Yi (mind). Double weight in another word is opposing force vs force.
Now that you know what is double weight is not opposing, double float is the opposite, i.e. using too much softness until you lost your body or stance structure and easily defeated. Bu Diu Bu Ding (not losing and not opposing) is precisely refering to these two deficiencies. Tai Chi is about Yin Yang interplay and inter exchange. Within Yin must have Yang and within Yang must have Yin.
