It is the attainment of a state in which opposing energies are present in equal strength. It is not necessary to be still to be balanced. Think of the magician who balances spinning plates on top of sticks. As long as they are spinning at the appropriate rate, they remain balanced. When they slow down, they become unbalanced, and fall.
Balance exists in motion.
Try standing on one foot. You will notice that your arms and legs adjust, bringing you in to a state of balance. You may be moving your arms or lifted leg in different directions, continuously adjusting your weight to remain balanced on one foot. At each moment that you remain on one foot, you are in balance. And yet the weight of your body is constantly adjusting, attempting to remain in position.