The seat (asana) should have the dual qualities of steadiness (sthira) and lightness (sukha). These qualities can be achieved by observing the reactions of the breath and body. From there the individual's dualities cease to disturb. Upon perfect stillness of the seat the uncontrolled and disturbed movements of the breath reduce (pranayama). The internal and external fluctuations of breath become soft and subtle when the transition (sthamba) between each breath becomes increasingly measured. The breath may then transcend consciousness (the thinking mind).
~YSP, 2:46, 47, 48, 49, 50, 51