The Hathayoga Pradipika (Ch. 3.55, 3.56) states that by practising the Uddiyana Banda, the prana rises through ths Sushumna, which is the central channel for energy flow. The Sushumna, is surrounded by two other channels or nadis called the Ida and the Pingala. When the energy or Shakti, rises through either of these, there is imbalance and hence rising through the central nadi is balanced.
A Banda means a lock. You are locking the energy into a particular center or part of the body. In Uddiyana, you are contracting the abdominal muscles after expelling the air. The vacuum created helps lift up the Kundalini that resides dormant in the root chakra (Mooladhara). Uddiyana means ‘to rise up’ or ‘fly’. The Sushumna is directly connected to the Sahasrara Chakra or the Thousand Petalled Lotus in the crown of the head, hence this rising through the central channel aids one is realization.
Uddiyana involves lifting up (and sucking in) of the abdominal muscles. It can be performed in a seated posture such as Padmasan or in a standing postures while resting the hands on the thighs, back stooping to the front. It should be done on an empty stomach, no exceptions.
It is easier to practice Jalandhara Banda (Throat lock) along with Uddiyana. In the throat lock, you press the chin into the base of the throat. Here you will be attempting to let the energy stay in the throat and hence vitalizing the Throat Chakra or Vishuddhi chakra.
Anyone with stomach or intestinal ailments, high blood pressure, heart disease, glaucoma or any kind of pressure in the head must not practice Uddiyana.
Swami Mukthibhodananda in his treatise of the Hatha Yoga Pradipika, states that with regular practice, vitality increases as Uddiyana has a powerful toning effect on the visceral organs, muscles, nerves and glands. The suction stimulates blood circulation and adsorption. The heart is squeezed and gently massaged. The suction draws up blood from the abdomen into the heart and at the same time, arterial blood is drawn into the internal organs. The processes of digestion, assimilation and elimination are directly affected. Uddiyana pulls the apana vayu up from the abdominal and reproductive organs towards the chest. Through Uddiyana and Jalandhara the prana is carefully locked into the navel region where union of prana and apana with samana can occur and induce the awakening of Kundalini. With so many beneifts, it is not surprising that uddiyana can slow down the natural process of ageing. The sloka (3.59) states after six months of practice death can be averted. Don’t take this literally, but realize the importance placed on the practice.