Bandhas are very important part of hatha yoga practice.
According to Hatha Yoga Pradipika, there are 4 limbs of Yoga and the third limb is mudra & bandha. According to Gheranda Samhita, there are 7 limbs of Yoga and bandhas are explained in the third limb in the subject of mudras. So, we can see how much bandhas are important according to the texts.
Bandha is a Sanskrit word that means to “hold”, “tighten” or “lock”. This define precisely the physical action involved for the practice of the bandhas. Specific parts of body are gently yet powerfully contracted and tightened.
This practice definitely required a proper guidance and lot of precautions. First, your body should be purified nicely by the Shatkarmas, controlled by Yamas, Niyamas and Asanas.
Then, you should have a control over your physical body, muscles, organs & nervous system. You should not have any kind of physical and mental ailments. After that, if you practice the bandhas, then their effects will be felt clearly in your physical body, pranic body & mind also. You would able to observe clearly the different sensations in your physical body, pranic flow in entire body and how the bandhas are affecting the mind, thought waves.
With the practice of bandhas, you will clearly experience how the mind is tranquilized and becomes receptive for the higher practice. So, before the bandhas preparatory practices are necessary to do...
Yogic texts says about the three granthis named the Brahma, Vishnu & Rudra granthi.
Granthis are the psychic knots. These psychic knots are considered as barriers or blockages of the path of sushumna nadi. Because of these psychic knots, kundalini energy cannot flow freely through the spinal column (sushumna nadi).
When they are pierced/broken, prana immediately begins to flow through the sushumna nadi which leads to increased receptivity of the mind that help for the higher experiences.
These granthis are located in the following part of the body:
Brahma granthi: pelvic region. It controls mooladhara and swadhisthana chakras and creates fear and insecurity, attachments to material objects, sensual pleasures and selfishness.
Vishnu granthi: heart region. It controls manipura and anahata chakras and creates bondage to other people, emotional relationships and attachment to emotional situations.
Rudra granthi: center of head region. It controls vishuddhi and ajna chakras and represents the obstacles created by the power of the intellect, along with attachment to siddhis (special power from the universe) and other psychic experiences.
Ultimately, the purpose of moola bandha is to unlock the brahma granthi, the purpose of uddiyana bandha is to unlock the Vishnu granthi, and the purpose of Jalandhar bandha is to unlock the last knot, rudra granthi.
So, there are 3 bandhas: jalandhara, uddiyana and moola bandha. But, while you practice all 3 bandhas together, then this is considered the fourth bandha called maha bandha (superior of all bandhas).
You can read the articles about each bandhas.