Mula Bandha [Root Lock]
Mula bandha is one of the three "locks" used in hatha yoga to locally contain the flow of energy or prana. This gesture consists in the permanent upward contraction of the anus sphincters. It is similar to Ashvini Mudra except that the contraction is constant while in Ashvini Mudra, the contractions alternate with relaxation.
The area is rich in structures (nerves, blood supply) and is directly linked to the spine. By activation of this area, the yogi promotes the activation of the whole nervous system. When this gesture is adequately done, the energy stimulated in this way can show up to the eyes.
This gesture also has the property to reverse the flow of Apana Vayu that governs the energies downward. Instead of naturally flowing downward and disperse, the breath is then directed upward and can fuse with Samana (see also concepts on breathing), the breath of the belly.
Through the mula bandha, prana and apana, nada and bindu unite and obtain perfection in yoga. There is no doubt about this. Hatha Yoga Pradipika
With practice the yogini will be able to sense the contraction of successive layers of muscles from the outside in. There are parts which can be differentiated: The contraction of the external anal sphincter, later the contraction of the second muscular ring and last the elevation of the anus. Play with a maximum contraction and hold it, as to really feel what is going on. When you are holding the bandha for extended periods you may be holding it gently at 15 -50% of your maximum.
Then, starting superficially and with a minimal effort, one can feel activity in the ischiocavernosus, bulbospongiosus and superficial transverse perineal muscles.
And with a little more attention one can activate the deep transverse perineal muscles and the urethral sphincter. And with yet more effort one can activate the pelvic diaphragm.
The central tendon of the perineum, which is located at the dividing line between the anal and urogenital triangles, appears to be the key structure around which the more delicate versions of mula bandha are organized. This is an extremely tough fascial region into which the superficial and deep transverse perineal muscles insert.
If the yogini can learn to focus his or her attention on this tiny region while creating minimal physical contraction of the nearby muscles, he or she will be feeling the root lock.
Uddiyana Bandha [Abdominal Lock]
With the lungs empty, one must engage in a fake inspiration, while blocking the breathing. This leads to the contraction of the belly. The energy of the belly is first led upward. Next, if Samana, the belly vayu burns with intensity, it also affect Prana, the vayu associated with the heart and emotions.
Jalandhara Bandha [Chin or Throat Lock]
Without bending over, one must push the chin towards the sternum while contracting the tongue and the throat. The chin does not necessarily have to touch the sternum initially. If Udana, the vayu of the throat is activated, it gives an ascending direction to all the others.
Jiva Bandha [Tongue lock]
The tip of the tongue is placed at the roots of the upper teeth and a mild contraction of the tongue is established.
Tria Bandha [The three locks]
The three locks are held together. The sequence depends on the posture. Generally, Shambavi Mudra is also established to promote an ascending direction to the whole technique.
In this way, the downward Apana is reversed, and fuses with Samana, the abdominal vayu. This in turns leads to the fusion of Samana with the heart Prana. Finally, Udana, the throat vayu becomes involved as well.
Vyana, the diffusive vayu, is then activated. It transforms into heat and shivers go through the body.
If the energy condensates in the spine and becomes pure again, it then resembles to an ascending movement
Note: For these operations to occur, one can see the necessity to prepare each center and insure it adequate functioning.