Perhaps no single phrase better sums up the art of yoga. When the mind ceases its wandering and constant movement (its 'vrttis'), when it rests in stillness, all that remains is the essential nature of reality: freedom and bliss.
This is easier said than done. The mind is normally in a constant state of change, identifying with the ever-adjusting external reality that is defined by physical space and time. Due to the immense difficulty of this task, the practices of yoga developed to aid in the taming and eventual stilling of the mind.
Early practitioners of yoga discovered that consciousness, 'citta,' and the energies of life, 'prana,' are intimately linked. The ways in which prana moves, citta will follow, and vice-versa. Far easier than controlling the fickle mind is grabbing the tail feathers of consciousness—prana; steadying the diffuse energies, and watching as the mind follows suit and becomes still.