The practice of yoga is referred to as sadhana. It is said that the essence of sadhana is the act of surrendering to the universal flow. In other words, in order for energy (Vital Life Force, Prana, Chi etc...) to properly course through the human body and generate the purification and cleansing processes of the autonomic nervous system, the body must be an open channel surrendered to the flow.
In hatha yoga, which aims to first purify the human body, consequently leading to purification of the mind and Spirit, achieving sadhana is necessary to awaken the chakras (energy centers) within the body. Specifically, hatha yoga focuses on the trinity of asana (yoga postures), pranayama (breathing exercises) and mudra (hand gestures) to awaken and focus this energy. When the trinity flows as one, the student will be awakened.
Asana, which is characterized as one of the original eight forms of yoga (and the only branch integrating physical movement), puts great emphasis on the directional movements of the spine. Accordingly, there are four of these movements: flexion (forward bend), extension (backward bend), lateral flexion (side bending) and rotation (twisting of the spine). To keep the energy flowing from pose to pose in asana yoga, the spine must be moved in one of these four directional movements or be at its core alignment position.
Pranayama is also considered one of the eight original forms of yoga, the principles of which are incorporated into all forms of yoga. Pranayama is all about the breath - and in the case of asana, pranayama is achieved by synchronizing the breath with the four directional movements. To allow the flow of pranayama, the goal is to extend the breath as long as is comfortably possible. By doing so, life energy is increased (This is no myth, for it is scientifically proven that animals who take more breaths per minute live shorter lives than animals who take fewer breaths per minute). The inhale/exhale duration ratio is 1:1 during flowing postures (movements), while the length of the exhale should be twice as longer than the inhale in sitting postures.
Mudra, or hand gestures, help to seal the energy within the body when they are synchronized with asana and pranayama. Think of mudras as being directors of the energy that is being generated and flowing within the body. There are many different mudras used during yoga - the two most popular being the prayer position and the jnana, which is the commonly depicted mudra with the the thumb and index fingers forming a circle.