Sri Aurobindo translates Mundaka Upanishad, Chapter 3, Section 1, Verse 9: “This self is subtle and has to be known by a thought-mind into which the life-force has made its fivefold entry: all the conscious heart of creatures is shot through and inwoven with the currents of the life-force and only when it is purified can this Self manifest its power. (The verb vibhavati seems here to have a complex sense and to mean, ‘to manifest its full power and pervading presence.’.)
The prana, or life-force, manifests in the human being with 5 major movements, which operate the major actions of the breath, inhalation and exhalation, as well as eliminative functions, and pervasive functions which operate to move energy through the entire body and to balance the actions of the pranas. The purification of the life-force is required as these pranas are mostly out of balance in some form or another and are driven, through desire, attachment, greed, fear, anger, etc. to great disturbance, which affects the ability of the mind to achieve the serene status of quiet needed to perceive the subtle Self that pervades the entire being and constitutes it.
It is a central tenet of Raja Yoga that Prana represents the link that infuses life into the body and activates the mental powers. This is the universal energy in its individual action. Therefore, by gaining control of the action of Prana in the body, one can gain control over the status of the mind and bring it to that state of quiet receptivity needed to perceive the Self. This led to the development of the practices of Pranayama and is a key element of the spiritual practice of countless seekers throughout the ages.
Purification of the life-force involves the quieting, balancing and harmonising of the action of the five major pranas. When one begins the practice of pranayama, one may observe all the nervous action at the physical level, but one also begins to see the influence of the impacts of the outer world on the mental status. It is the prana that takes on the coloring of the emotions and vital desires and physical needs and all of these things create ripples of energy in the mind-stuff. Yogis recognize that each nervous or emotional status has its own characteristic of breathing, and by consciously controlling the breathing, one has access to bringing the reactions under control. Thus, it is possible through pranayama to prepare the mind-stuff for the serene and glad quiet that is necessary for the Self to manifest its native power without the deformations of the body-life-mind complex.
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210