As I'm sure you already know, yin and yang are more than a simple way of comparing and organizing things of the universe. It's also the fundamental understanding of how the universe works. The white represents yang: energy that rises, associated with the sun and day, activity, and the male gender. The black represents yin: energy that descends, associated with the moon and night, rest, and the female gender. The larger portions of black and white are representative of balance and harmony, and the dot of the opposite color in each half represents their root of existence. Let's start with balance and harmony. Think of a relationship in your life. If you're with someone whom you consider your equal, there's little conflict and what differences you do have can be used to strengthen your bond. If you're with someone and there isn't a balance of power, turmoil ensues (and we've all been there). The same logic holds true for everything else in the universe and it's where we acupuncturists do some of our work- harmonizing the relationships between organs so everything functions as it should. Now to explain the root of existence. Yin cannot exist without its root in yang and yang cannot exist without its root in Yin. Nothing can be entirely yin just as nothing can be entirely yang. If anything is purely one or the other, it actually creates separation and therefore death. To use men and women as another example, if one was to separate them from each other, the population would eventually go extinct. When we put those two aspects together, the symbol in it's entirety represents the transformative properties of yin and yang. Because each is rooted in the other, they have the ability to change from one form to the other, and continue to do so in an endless cycle shown by the buta-like shapes made with the larger black and white portions. We see this transformation every day as it changes into the night, or when children (yang) evolve into the elderly (yin).
A lot to be said of such a simple symbol, right?