What effect does hatha yoga have on the body and mind?
Swami Sivananda: By the process of hatha yoga the yogi attains a perfect physical body. Balavajrasam-hanana kaya sampat – “The perfection of the body consists in beauty, grace, strength and adamantine hardness.” The power to bear extreme cold and heat, the power to live without water and food and other powers come under the category of kaya sampat, perfection of the body. Since the body of the hatha yogi is perfect and firm, his mind is also firm and one-pointed. By practising dharana and dhyana he reaches the highest rung in the yogic ladder and attains immortality through yogic samadhi. The yogi who has reached the highest stage, will have the eight major and all the minor siddhis. This is stated in Hatha Yoga Pradipika (3:8):
Vallabham sarvasiddhaanaam durlabham marutaamapi.
Adinath said the mudras and bandhas are the bestowers of the eight divine powers. They are held in high esteem by all the siddhas and are difficult for even the gods to attain.
I have seen many Vedantins who are in a sickly condition with a very poor physique. A Vedantin is afraid to do asanas and pranayamas on the grounds that their practice will intensify body awareness, and adversely affect the practice of vairagya. Although the two paths of hatha yoga and Vedanta are different, a Vedantin can harmoniously use pranayamas and asanas to great advantage. The body is closely related to the mind. A weak, sickly body also means a weak mind. This body is a horse to take one to the goal. The body may be inert and useless but it is an important instrument for self-realization. So it must be kept clean, strong and healthy. The Vedantin who practises a little hatha yoga to keep his body and mind healthy and strong is capable of doing very good sadhana and reaching the goal of life quickly.
Swami Sivananda: Unfortunately, yoga is more or less regarded as a form of physical culture divested of all its psychological and spiritual significance. This is due to an exaggerated emphasis on the yoga asanas. Hatha yoga practices do form a preliminary background for progress in yoga, but they are not absolutely indispensable for yoga sadhana. While a strong body is an asset, a weak body is not always a handicap. There are instances in the history of yoga where the ultimate has been achieved while dispensing with the practices of hatha yoga. Nevertheless, I believe that everybody should do asanas regularly. One of the aims of this yoga movement is to popularize the use and the benefits of asana in the daily life of men, women and children. Hatharatnavali (1:22), states:
Yuvaa bhavati vriddho vaa vyaadhito durbalo'pi vaa;
He who untiringly practises yoga in all its aspects attains success even if he is young, old, diseased or weak.