Bandhas & Mudras
This is an esoteric part of yoga practice. I have very little exposure to bandhas and mudras; hence I will hand you over to my teacher, Shri C S R Prabhu, who has over 40 years experience in Hatha yoga.
The word Bandha means "the lock" and, as per the Hatha Yogic literature, there are three fundamental bandhas: Moola Bandha, Udyana Bandha and Jalandhara Bandha.
This deals with the rectum, anus,. You control the lock by breath control either in exhalation or inhalation, with retention of breath, i.e. after you exhale you retain or after you inhale you retain. Moola Bandha can be practised in both options. In Sukshma Vyayama, it is practised only in the inhalation option, i.e. you inhale and pull up your rectum and anus after pulling up the buttocks, thighs and legs. In traditional Hatha Yoga, however, the same Moola Bandha is performed in Padmasana both in the inhalation retention mode or exhalation retention mode.
This is complete exhalation followed by a deep contraction of abdominal muscles (commonly called rectus muscles) and holding them up without breathing. It is also called Udya Bandha or Ajagara Mudra. It is said the Ajagara or the python, exhales and creates a vacuum and sustains that retention for a long time. When it sees the prey, it releases the retention. There is a great force of air that causes the prey to run into the python's mouth after which the python closes its' mouth for good. It is also called Udyana Bandha because of the exhalation and retention after the contraction of the abdominal muscles. This is a profound mudra. It can overcome digestive problems, increase appetite, reduce fat in the abdominal muscles, help in gastritis and diabetic conditions, activate the pancreas and control the liver. A large number of activities related to digestion can be regulated and controlled by Udyana Bandha, which is the central point of Sukshma Vyayama - Udara Sakthi Vikasika. The abdominal exercises start with Udyana Bandha. We perform abdominal breathing in different positions and different postures to get maximum benefit.
This is a "chin lock" which is done in exhalation retention or inhalation retention mode. The retention is done with a chin lock with closed eyes. The chin is pressed against the chest, with the eyes closed, with retention of the breath either after exhalation or after inhalation. This is called Jalandhara Bandha. This Bandha has got tremendous effects on the heart, lungs, brains, concentration power and the eyes. It is dangerous to open the eyes when Jalandhara Bandha is performed as eyes will be affected negatively and damaged. So Jalandhara Bandha becomes the central point in half a dozen Sukshma Vyayama exercises.
Sankaracharya says "he who practices the three Bandhas, where is the Bandha for him from the deadly noose of the time Kala Pasa, sloka again that female snake in the bottom of the abdomen will be woken up by the 3 Bandhas and goes up into the higher levels of the body and absorbs into higher levels of consciousness like Samadhi and in fact the snake is nothing but the Kundalini Sakthi in the body. It is called Kundalini because it is wound up like a snake, it has got nothing to do with the snake and it is only a figure of speech and Jalandhara, Moola and Udyana Bandha will be awakening the snake which is dormant and sleeping in all the human beings and is awakened in enlightened human beings
You can read more about Yoga Mudras and their with scientifically proven benefits here