After you have mastered the energy locks you are ready to incorporate them into more complicated techniques that are the core of tantric practice: Aswini Mudra (AhswiniMudra अश्विनी मुद्रा) and Vajroli Mudra.
These practices both generate intense energy in the lower body while keeping it contained. This builds up pressure in the energy channels. Once the containment is released, that pressurized energy shoots up the spine, breaking through any blocks or restrictions.

Ahswini Mudra Aswini Mudra अश्विनी मुद्रा

Ahswini Mudra (gesture of the horse) — The word Ashwini means ‘Horse’. The practice is so-called because the anal contraction resembles the movement a horse makes with its sphincter immediately after evacuation of the bowels. This is a very powerful technique to pump the energy up into Manipura. This asana involves the contraction and relaxation of the buttock muscles, sphyncter, perineum, and the entire pelvic region.

There are two methods of practicing Aswini Mudra,
Aswini Mudra (version 1)

Position of readiness
Sit in any comfortable meditation asana.

Steps
Close the eyes and relax the whole body.
Become aware of the natural breathing process for a few minutes, and then take the awareness to the anus.
Contract the sphincter muscles of the anus for a few seconds without straining, then relax them for a few seconds.
Try to confine the action to the anal area. Repeat the practice for as long as possible.  Contraction and relaxation should be performed smoothly and rhythmically. Gradually make the contractions more rapid.

Aswini Mudra (version 2) Rapid contraction

Position of readiness
Sit on your heel or on a rolled up towel or tennis ball so pressure is felt on Muladhara Chakra (between genitals and anus for men, on clitoris for women).

Steps
Inhaling fill the lungs about 1/3 total capacity of lungs.
Contract and relax the anus 15 times, about twice per second.
Exhale
Again inhaling fill the lungs about 1/3 total capacity of lungs.
Contract and relax 15 more times.
Exhale
Again Inhale the last time fill the lungs about 1/3 total capacity of lungs.
Contract and relax 15 more times.
Hold the breath, pull the shoulders forward and press the chin against the chest for a moment. Feel the heat and hydraulic pressure build up.
Take a sniff of air and exhale through the nose as you release the tension. Feel an energy flush through the entire body. Visualize the energy moving up the spine.

Benefits of Ashwini Mudra

This Mudra has particularly good effects on the empty spaces in the stomach. Due to the pressure functioning of these parts is improved. It also has good effects on Uterus in women.
It tones up pelvic muscles, nerves surrounding the anus sphincters.
Very useful in rectum and anus diseases.

Note
It is one of the most important exercises for preparing the mother for childbirth. It is performed with the knees slightly apart, and the forehead resting on the arms folded on the ground.
Any healthy person can practice this asana but women suffering from Uterus problems should do this asana under expert observation.

Version 1 is recommended to pregnant women (The Ashwini mudra for contraction and relaxation of the pelvic muscles can also be practiced while lying on the back with the knees bent). Regular practice of this Mudra exercises and strengthens the muscles of entire pelvic region. It helps the mother cope with the weight of the child during pregnancy and eases childbirth by virtue of the fact that the mother is perfectly trained in relaxing the muscles of the vagina. The woman in labour creates no resistance, and therefore saves herself pain and fatigue.

Bandhas are necessary for practice of Pranayama, Bandha means energy lock. There are main 3 types of locks, first is Jalandhar Bandha or Chin lock, second is Moolabandha or anus lock and third is Uddiyan Bandha or abdominal lock. These locks are essential while doing the Kumbhaka that is retaining the air inside or outside of the lungs.
During Pranayama, these three Bandhas are used.