The concepts of sun and moon elicit in us a reference to masculine and feminine. Hatha yoga, is about disciplining the body (and mind)'s energies. The masculine energy is linked to the God Shiva, and is called the Prana (masculine). The feminine energy is linked to the Goddess Shakti (Shiva's consort) and is called the Aapana. Together they form the Kundalini, which is like a spiral of energy that flows the length of the human spine. This accounts for the focus of Hatha asanas on the spinal column - it's also why it is important to perform poses on both sides of the body (balance).
In theory when we practice Yoga we are trying to join the masculine and feminine energies of our body, thus becoming 'whole' and achieving "a poise of the soul which enables one to look at life in all its aspects evenly." (Mahadev Desai as quoted by Iyengar in 'A Light on Yoga'). These energies flow through our subtle body (our non-physical or psycho-spiritual body... it's complicated - look it up!) by means of channels called nadis. The nadis run all along the body, connecting at 6 special centres of energy called chakras. But now I'm getting sidetracked.
I recently parcitipated in an Anusara Yoga workshop in which the Teacher discussed this balancing principle. He discussed masculine energy as muscular energy. It is that energy which is powerful, energetic, and giving. When you push up from plank pose (kumbhakasana) to downward-facing dog (adho muka svasana), that would be masculine energy. But once you arrive in the pose, you invoke your feminine energy to soften the upper back and the shoulders and sink gracefully into a deeper stretch. The feminine energy is what allows us to be creative, countering the strong but rigid masculine energy with a gentle touch that says "what if...?".
What amazes me is that in my 7 years of practicing Yoga, I have only just discovered this. How did I miss it? It's fascinating (to me!) that in Western Yoga, which is so female-dominated, the feminine principal of Yoga seems to play second fiddle. Is this because the main styles of Yoga we practice today were male-initiated? Or is it because Western society is full of those rigid, energetic masculine principles? Because we are so focused on the individual, or on attaining instead of letting go? One example is our typical Yoga mat - straight and narrow. Why did it take me 7 years of Yoga to hear a Teacher say: "go ahead and go outside your mat". Simple, yet it can change the whole way you practice Yoga. It feels like coming home.
From this we learn a valuable lesson. Yoga is neither masculine nor feminine but both. It is strong yet soft, rigid yet fluid, it is fixed in a moment but flexible and changing always. Somewhere in there is a balance - a moment when time stops, when the ego dissolves, when the Yogini or Yogi just is. And that is Yoga.