A yogi is in reality a master of the three gunas. He gets the ability to expel and employ darkness, ignorance, passions, actions, ambitions, purity and enlightenment. They come to him at will, when he wants, and do not come when he does not.
Sri Krishna assures Arjuna that only such a person can ever hope to be able to be united with the eternal spirit and the divine source of the universe, through development of perfect understanding of the deep significance of the three gunas, who worships the Lord and performs bhakti and the yoga associated with it, in pureness of heart and mind.