(without cosmic consciousness)
Many years ago, when I was "shopping around" for a meditation practice that I could do regularly, I found a teacher who claimed he had the best meditation practice to offer. It was a mantra based meditation. When I asked him how it differed from other mantra based meditations, he told me that his technique worked at the level of cosmic consciousness.
Similarly some teachers, when asked by their students about bandhas, just tell that it a very subtle feeling that has to do with the chakras and the kundalini, leaving the already perplexed student, even more so by being bombarded with lofty language and ideas they do not understand or have a feel for. While it is true that some of the internal bandhas like the moola bandha take years of regular practice to get a hang of, the general concept of bandhas is straightforward and deserves a simple explanation to any seeker.
1) What is a bandha ?
The word bandha means constrain, bind or lock. But what is the entity that is constrained, bound or locked. To understand this better, let us take the example of the final version (version D) of marichiasana shown above. This is a deep twist and it comes in the Ashtanga Primary series. The hands wrap around the right knee and the body, while the left foot is tucked between the right leg and the torso. In this posture there are no loose body parts that flap around and squander energy. The entire body is self contained and intact. One can then say that there is a bind on the body preventing squandering of energy. In this state, the attention is brought to lengthening the spine. It gives a feeling of energy rising up the spine. In yogic terms, the prana is channeled to flow along the spine. Additionally, people use the moola bandha (an internal lock, discussed next) to enhance this effect.
2) Effects of the bandha on the mind and the body
This effect of channeling the prana to flow along the spine is akin to focusing the rays of the sun using a lens. The positive effects this has on the mind are:
1) focus, because the mind is focused on raising the prana along the spine,
2) states of joy and happiness, caused by the rising prana
3) A surge of energy in the body by chaneling the prana to move in a certain direction
3) External bandhas
The bandhas in the body can be external or internal. Simply any act that binds the body, avoids unnecessary movement and channels the prana is a bandha.
For example, in Utthitta Trikonasana (shown in the picture below), when the front foor is pushed forward and the rear foot is pushed backward, there is friction created between the yoga mat and the feet which gives the feeling that the feet are rooted to the ground. With this strong fundation, one is able to divert the energy upwards to the raised hand. Here the external bandha that is used is 'Pada Banda' or the foot lock. Similarly, there is also a hand lock.
4) Internal bandhas
The three most important internal bandhas are the Moola bandha (root lock), the Uddhiana bandha (the diaphragm lock) and the Jalandara Bandha (chin lock). Of these the moola bandha is the most important. More about moola bandha can be found in the slides and video recording of the workshop tilted "Finding moola bandha".
References to the other bandhas can be found in the slides and recording of another workshop titled "The subtle aspects of Hatha Yoga".
1) “Finding Moola Bandha” workshop slides and video
2) Moola Bandha : Excellent book that talks about the endocrinological and neurological benefits of Moola Bandha