By Yoga Swami Svatmarama
An excerpt of Iyengar's foreward:
"If Patanjali, in the Yoga Sutras, codified the eight limbs of yoga (astanga yoga), Svatmarama did the same for hatha yoga.
» Hatha means willpower, resoluteness and perseverance; and Hatha yoga is the path that develops these qualities and leads one, towards emancipation. The word hatha is composed of two syllables: ha and tha. Ha stands for the seer, the Self, the soul (purusa), and for the sun (Surya) and the inbreath {prana). Tha represents nature (prakrti), consciousness {citta), the moon (chandra) and the outbreath (apana). 
» The Pradipika is divided into four parts. The first explains yamas (restraints on behaviour), niyamas (observances), asanas (posture) and food. The second describes pranayama (control or restraint of energy), and the shatkarmas (internal cleansing practices). The third deals with mudras (seals), bandhas (locks), the nadis (channels of energy through which prana flows) and the kundalini power. The fourth expounds pratyahara (withdrawal of the senses), dharana (concentration), dhyana (meditation), and samadhi (absorption). 
» In all, the text contains 390 verses. Out of these, about forty deal with asanas, approximately one hundred and ten with pranayama, one hundred and fifty with mudras, bandhas and Shatkarmas and the rest with pratyahara, dharana, dhyana and samadhi. Asanas The text begins with asanas as the first step in hatha yoga.
» However, hatha yoga does not overlook the yamas and niyamas. He does speak of non−violence, truthfulness, non−covetousness, continence, forbearance, fortitude, compassion, straightforwardness, moderation in food and cleanliness as yama, and zeal in yoga, contentment, faith, charity, worship of God, study of spiritual scriptures, modesty, discriminative power of mind, prayers and rituals as niyama. 
» Asanas, pranayamas, bandhas, mudras and shotkarmas are illustrated by examples to assist aspirants with their practice. It is said that there areas many asanas as there are living species: 840,000. That means the muscles and joints can flex, extend−and rotate in several thousand ways. The Pradipika, however, describes only sixteen asanas. 
» In view of these figures, to claim that hatha yoga is merely physical yoga is simply ridiculous. Yogis were in constant contact with nature and they were searching for natural remedies to combat afflictions. In their search, they discovered hundreds of asanas to increase the life−giving force and restore it to its optimum level. Asanas are not just physical exercises: they have biochemical, psycho−physiological and psycho−spiritual effects. The cells of the body have their own intelligence and memory. Through practice of different asanas blood circulation is improved, the hormone system is balanced, the nervous system is stimulated, and toxins are eliminated, so that the cells, sinews and nerves are kept at their peak level. Physical, mental, and spiritual health and harmony are attained. 
» Patanjali states that perfection in asanas brings concord between body, mind and soul. When asanas are performed with the interpenetration of all three, benevolence in consciousness develops. Then the aspirant ceases to be troubled by the pairs of opposites, and the indivisible state of existence is experienced."