A Bandha is an energetic lock, helping to lock prana and energy in a specific area of the body, helping to send energy to a specific location. There are three major Bandhas, Mula Bandha (root lock), Uddiyana Bandha (means flying upwards, upwards lock, around the navel), and Jalandhara Bandha (the throat, literally translates as a water net and supporting).
“Through this bandha, the great bird “prana” constantly flies up through the Sushumna nadi; that is why it is called Uddiyana (upward flying).”
— Hatha Pradipika 3:55
From Mula Bandha lifting towards the solar plexus, Uddiyana Bandha happens, another magnetic location right around the navel. There are two types of Uddiyana, the full vacuum of abdominal walls, and the one we practice in Asana which is much gentler.
The Full vacuum practice is considered even a full kriya: it is done at the bottom of the exhale, there is no inhale, there is a mock inhale as if you were inhaling you lift everything upwards. Usually practiced standing up, hinging forward, you exhale open mouth fully until there is nothing left in the lungs, then using your hands on knees or thighs to lift everything upwards. Your belly is relaxed, your muscles do not flex.
Before you attempt this practice though be sure you have an empty stomach!
The regular practice of this Kriya increases blood circulation throughout your body and to your brain, strengthens internal organs and the vagus nerve, which helps activate your parasympathetic nervous system, turning on your rest and digest response and bringing a calmed state of being.
“Even an old person can become young when Uddiyana Bandha is done regularly.”
— Hatha Pradipika 3:58
Enjoy your pranayama practice!