The movement of breath and energy in the body
There are three primary bandha in the body that are used in hatha yoga practice. They are Mula Bandha, Uddhiyana bandha and Jalandhara bandha.
Mula bandha is located in the area of the root chakra, and physically represents the pelvic diaphragm that is interior to the genitals on pelvic floor. To start to feel mula bandha, sit in easy pose. Try to locate the sitting bones in your awareness, which you can do with a little wiggle. Then check that you are on the sitting bones, so that the pelvis is upright. (If not, the spine will be curved, and it is difficult to feel the movement of the pelvic floor). Take a few deep breaths and allow movement of the abdominal muscle wall. As you take these breaths, take your attention to the pelvic floor. On inhale, there is a natural expansion of the musculature in the pelvic floor, as if the root was stretching wider and moving down towards the earth. Taking those same deep breaths, start to resist that opening. Instead begin to pull up on the part of your body that was moving down. This is not the way the pelvic floor is designed to move with breath and so it creates a kind of restriction in the body. Can you feel your breath as deep? Do you feel any other sensations in the body?
Mula bandha is neither the tightening of the anus, nor the sensation one would have if stopping urination mid-flow. It is creating something like a trampoline with the pelvic floor in the diamond created between the two sit-bones, the pubic bone and the tailbone. With that energised and aware diaphragm, the trunk of the body is supported against the pull of gravity. Energy is contained in the body, as if it is being bounced back up. The toned pelvic floor is like a surface from which kundalini energy can rise. Mula bandha is a challenging, esoteric and controversial practice. I have written more about it here.
Uddhiyana bandha works in conjunction with Mula bandha and is located in the lower stomach area. It literally means flying up and is the lift and scooping up of the lower abdominal muscle wall. Before birth, we were fed through the umbilicus into the stomach, and so this is a powerful place where energy enters the body. From the navel point, energy descends to the root, where it is contained by mula bandha. It can then start to rise through the chakras until ascension to the crown. Uddhiyana bandha is not the tightening of the abdominal muscles like one would do in a sit up or if (as children might do) one was trying to show off strength by taking a punch to the stomach. In fact it is quite the opposite.
Uddhiyana translates as flying up. Between the two hipbones in front, there is a hollow that lifts up. One way to feel this passively is to lie on the back with the soles of the feet on the floor. The tailbone is not tucked, but instead the back surface of the pelvis pushes down into the earth. With that plugging down, the navel draws in towards the spine, without moving the spine or the pelvis. This creates a hollow between the hipbones, which is the flying up of uddhiyana bandha. Between the pelvic floor in mula bandha, and abdomen in uddhiyana bandha there is a cycle of energy. In through the navel point, down through the tailbone, up through mula bandha and further lifted in and up through uddhiyana bandha. (These external body locations are only partially accurate as the energetic cycle is internal to the body and not necessarily located in the particular bones and organs.)
Jalandhara bandha is the neck lock, and is located interior to the body in the space behind the collarbones. If we think of the torso and abdomen like one unit, sealed off at the bottom by mulabandha, the jalandhara bandha seals it at the top. It is like a gate that serves to contain energy so that it does not rise to quickly into the head. In a light jalandhara bandha, the collarbones draw back into the body and the chin comes in towards the throat. This creates length in the back of the neck, supporting the head. In a more vigorous neck lock, which is used in breath retention, one can feel the gate at the base of the neck behind the collarbones drawing in and holding energy and breath down.