Yoga has an outer aspect, which entails of right living, right care of the body, and enhancement of vital energy. This is what yama, niyama, asana, and pranayama are all about. Yama and niyama build a foundation of right behavior through such values as nonviolence and truthfulness and such practices as cleanliness and contentment. Asana makes the body strong and flexible, and pranayama develops our vital energy.
Yoga has an inner dimension i.e meditation or the being with of higher consciousness. This is the real purpose of yoga, the focus of dharana, dhyana, and samadhi, which together form a single process—samyama, or meditation in the broadest sense.
As the fifth of the eight limbs, pratyahara inhabits a central place. Some include it among the outer aspects of yoga, others with the inner aspects. Both classifications are correct, for pratyahara is the key to the relationship between the outer and inner aspects of yoga; it shows us how to move from one to the other.
It is not possible for most of us to move directly from asana to meditation. This requires jumping from the body to the mind, forgetting what lies between. To make this transition, the breath and senses, which link the body and mind, first need to be brought under control and developed properly. This is where pranayama and pratyahara come in. With pranayama we control our vital energies and impulses, and with pratyahara we gain mastery over the unruly senses—both prerequisites to successful meditation.