The yin/yang symbol is one of the oldest and well-known symbols in the world. The traditional thinking involves two universal forces, representing the two poles of existence that are opposite but complementary, and which, exist in everything.
The symbol includes a small black spot in the white swirl, and a corresponding white spot in the black swirl. According to the philosophy, everything contains some yin and some yang. Nothing is totally of one pole as there is always darker or lighter.
The two swirling shapes inside the symbol give the impression of change--the only constant factor in the universe. In fact one of the few rules is as something reaches an extreme level it always becomes its opposite. This is illustrated in nature in many ways. It applies to all things: a storm is preceded and followed by a great stillness; cold replaces hot replaces cold in the constant cycle of the seasons; an organisation that is too strict will cause a rebellion when the rules become too tight; a balloon will burst if over inflated.
These examples seem trivial, but a person who can spot yin and yang operating in the world can predict the outcome of events. Hung Fut martial artists are made aware in training to try and maintain a balance. If excessive force is used, it can be easily turned against them by pulling in the same direction. The opponent can use your forceful attack, by using that energy, to cause a defeat. The student is taught to never over extend, always to keep limbs slightly bent and never fully straightened, and to mix the hard with the soft.
