marvin8 wrote:What are the benefits of opening and closing the kua in a martial context?
Your question doesn't really make much sense. Different styles use the body differently, including how they use the kua. Some styles use a stance that has a closed kua (e.g. Wing Chun). Some styles use a stance that has a predominantly open kua (e.g. Chen Taijiquan). Both work.
If your question is why would one want to
alternate opening and closing the kua - within the same style - again, the short answer is that it is part of how one, in that style, uses the body to generate force/power and to neutralize, how one coordinates - or not - the parts of the body. It is basic to the style and how the body is used within that style. In Chen Taijiquan, one of the simplest, overt examples of opening or closing the kua is to power a leg strike against an opponent's leg to destabalize the opponent's leg. An example of the action in solo training is shown at 1:57 in this video:
https://www.youtube.com/watch?v=oRAv85s3OZkOpening and closing the kua are independent of "sinking": they are two different things. One can open the kua and not sink, one can close the kua and not sink, or one can open the kua and sink and close the kua and sink. It also has nothing to do with "empty" or "full": one can have an open kua that is empty or an open kua that is full, etc.