The flow of prana in the nadi's is directly related to the state of mind. There are three major nadis - ida, pingala and sushumna. Generally the breath shifts between ida and pingala. For a brief duration, it flows through sushumna during the shifting phase. The mind becomes free only when the breath flows through the sushumna. Until then it remains captivated by duality, the good and bad, positive and negative, pleasure and pain... When the breath flows through sushumna, the mind becomes more of an observer. It is the state of meditation. The goal of a yogi is to keep the breath in the sushumna channel.