Imagine your body and mind like the string of a cello. The energy that lives in a string pulled from the base of the cello to its scrolling top can create a lovelyvibrating sound.
Same with you! Energetic balance in a yoga pose sounds rich and full. Pulled too tight, you might sound sharp, too loose could sound flat; the middle ground is where your body will vibrate right on key.
The words hatha yoga translate as sun(ha) and moon(tha) union(yoga). Too much sun would burn us; too much moon and we would be unable to grow, to wake up. The balance, or union of the sun and the moon creates life. The balance can be found in your tadasana (mountain) pose as well. Your feet root you to the floor. Your head is in the clouds. The balance of the two is what you breathe into to create life.
The opposites that can be balanced in yoga are endless: there is cool and warm, energy and relaxation, effort and surrender, remembering and forgetting. Yoga lies in between heaven and earth, liquid and solid, noise and silence, movement and stillness.
How do you find the balance between the opposites? By being very quiet and listening to where one element begins and merges into its opposite. That in itself is enough practice for a life time of yoga.