The ying and yang describes the realism of the universe. In diagram form, they are drawn as near to halves of a perfect circle. Which means they are inseparable by nature, in contributing to the completeness of energy changes in the universe.
Although superficially they are opposite to one another, however the expression of ying has to be stemming on the yang, and vice versa. For example, the softness of the Tai Chi Chuan can only be expressed by having strong muscles of the body for firm stance, and slow movement of the arms which brings much stress to the body. And the breaking power of the punch(yang) can be enhanced by creating acceleration at every joint(ying), from ankle to knee, to hip, to waist, to shoulder, to elbow and to wrist, then finally to the fist.
The crudest practicing of Wing Chun would be to learn to appreciate the expressing of body's ying and yang. Most martial artist stop progressing from that level onwards. Because the further development of the art can only be possible if the martial artist has developed his mind along the way. The enemy is no more a physical opponent, but the mind itself. The ultimate goal of the martial artist is to tame the mind. Hence, the mind is ying and our physical body is yang. The stronger is our mind, then so does our body, and vice versa.