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In the fourth chapter of the Bhagavad Gita, Sri Krishna instructs Arjuna on raja yoga and hatha yoga. His basic advice is: unite prana with apana or unite apana with prana (4:29):
अपाने जुह्वति प्राणं प्राणेऽपानं तथाऽपरे ।
प्राणापानगती रुद्ध्वा प्राणायामपरायणाः ॥ Apana is absorbed in prana (by some), and prana in apana by others. By restraining the courses of prana and apana, (the yogi is) wholly occupied in pranayama.
The union of prana and apana, or apana and prana, or the suspension of both, can be attained through the practices of yoga. There are many practices for bringing about this union. When siddhasana is practised and the inspired breath is taken to the region of samana, prana and apana unite at this centre. The practices of kriya yoga bring about the same effect. When the three bandhas are performed, jalandhara bandha stops prana from ascending, moola bandha stops apana from descending, and uddiyana bandha creates a suction process, which affects the behaviour of both prana and apana. A shatkarma like nauli kriya also creates union between prana and apana. When pranayama is practised with inhalation, exhalation and retention, the union of prana and apana takes place at ajna chakra.
The union of prana and apana is one of the most important aims of yoga. Until the practitioner is able to channel these two forces, he continues to be under the influence of duality and the mind remains dissipated. The Yoga Chudamani Upanishad says (v. 27):
आक्षिप्तो भुजदण्डेन तथा यथोच्चलति कन्दुकः।
प्राणापानसमाक्षिप्तस्तथा जीवो न तिष्ठति॥ Just as a ball goes up and down when thrown by the hand, similarly, the individual soul is thrown up (and down) by the movement of prana and apana (and therefore) does not remain still.
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