Emotions are like rocks we throw into the water, making it hard to see things as they are, making the moon seem distorted. Calmness is the key to Clarity. A knife is not in itself good or evil, but how people decide to use it. The fact that the knife exist brings opposite possibilities, yet if the knife did not exist, neither killing nor saving lives with it would be possible. Duality is not bad and is not good but is both, while void is neither.
Polarity is necessary for consciousness, in voidness there is no consciousness.
Now left and right can be seen as yin and yang, top and bottom can also be seen as yin and yang, while the middle or mid point is seen as the balance. In fullness one seeks the middle, in emptiness one need not seek, yet the ultimate balance is between fullness and emptiness, between form and space. So when one plays the trumpet, one has to find a balance between blowing way too hard or not blowing hard enough soft, yet to seek balance in emptiness, one simply has to not play the trumpet. This I see as balancing between left, middle, right and void. The goal of a Taoist is usually to find balance between left and right in form, as well as balance between space and form.