Prana is not simply a physical energy in the body. It is also the energy that operates in the mind. Swami Vivekananda described this subtler form of Prana as psychic Prana and indicated that it is the clue that provides the practitioner of Yoga the ability to gain control over the mind. By gaining mastery over the Prana, the mind comes automatically under control. The breath, acting as the most visible and manageable form of Pranic action, is the key. Thus arose the science of Pranayama as a stage in the systematic practices known as Patanjali’s Yoga or Raja Yoga.
Sri Aurobindo explains: “…by control of the Pranic energy it is not only possible to control our physical and vital functionings and to transcend their ordinary operation, but to control also the workings of the mind and to transcend its ordinary operations. The human mind in fact depends always on the Pranic force which links it with the body through which it manifests itself, and it is able to deploy its own force only in proportion as it can make that energy available for its own uses and subservient to its own purposes. In proportion, therefore, as the Yogin gets back to the control of the Prana, and by the direction of its batteries opens up those nervous centres (cakras) in which it is now sluggish or only partially operative, he is able to manifest powers of mind, sense and consciousness which transcend our ordinary experience. The so-called occult powers of Yoga are such faculties which thus open up of themselves as the Yogin advances in the control of the Pranic force and, purifying the channels of its movement, establishes an increasing communication between the consciousness of his subtle subliminal being and the consciousness of his gross physical and superficial existence.”
Sri Aurobindo, The Upanishads, Kena Upanishad and analysis, pg. 102, 156-160