There are some physiological principles on which the postures (asanas) of Hatha Yoga are based;

1. Circulation
   Rhythmic movement linked with the breath, as in the Sun Salutation sequence, stimulates the heart and lungs and circulation of blood and body fluids.
   The different positions of the body in the asanas exert a ‘squeeze and soak’ action, by which the organs and tissues are alternately compressed, squeezing blood out and then released, so fresh blood flows in.
   The action of gravity drains the blood and fluids from particular areas of the body and increases the flow to others. Inverted poses, for example help drain and relax the legs and enhance circulation in the upper body.
2. Stretching
   This releases energy held in the muscles as excess tone or tension, relaxes them, increases their range of movement and overall flexibility and helps reduce the occurrence of injuries.
3. Alignment
   Moving slowly and consciously into and out of postures, while taking care to keep the joints correctly positioned and working within a comfortable range helps correct muscular and skeletal imbalances. This improves posture, improves joint stability and reduces age related 'wear and tear'
4. Breathing
   Entering and exiting postures on the breath and holding them with slow, deep breathing regulates the breathing, increases the oxygen supply, blood circulation, energy (prana) levels and enhances physical and mental relaxation.
   Pranayama is the practice of various breathing exercises to enhance control of the vital energy through the breath.
5. Awareness
   Giving attention to the breath and to specific body areas engages the mind and increases the blood flow and prana supply to the target area, Focusing the mind on parts of the body builds self-awareness, increases mastery and control of the body and lays the groundwork for meditation and higher states of consciousness.
6. Relaxation
   In the conscious deep relaxation of 'savasana' (corpse pose) one systematically relaxes each part of the body releasing all the muscles and allowing the joints to open up. This leads into Yoga Nidra (yogic sleep) which thoroughly replenishes the body and calms the mind and the nervous system.In this process, we learn to observe the physical sensations, thoughts and emotions as ever changing currents of energy, which flow through us but are not the essence of who we are. Thus Yoga teaches us that consciousness and thoughts are not the same thing and helps to set us on the path to realizing the true nature of being.

When you know what you are doing, you can do what you want
~ Moshe Feldenkrais ~