The book, Moola Bandha, The Master Key, by Swami Buddhananda roughly describes what might be happening in the nervous system when we practice mulabandha.
When a yogi decides to practice mulabandha, something is happening in the brain—a thought that attempts to contact the south pole! Then the message the brain sends to do something at the base of the body, like activating some muscles “down there”, goes down the spine to the sacral nerves 2, 3, 4 (shown in the illustration above). The sacral nerves are in the low back region of the body. The sacral nerves then send messages to the muscles that contract the pelvic floor, if everything goes as planned. This is thought to affect the Parasympathetic Nervous System, and having the effects of decreasing blood pressure, respiration and heart rate.
The sensations of relaxation and pleasure generated by this practice are relayed back to the brain where they are analyzed by the higher mental faculties of the cerebral cortex (brain), and transmitted to the mind for appreciation and enjoyment.
~ Swami Buddhananda
The Parasympathetic Nervous System is controlled by the above-mentioned sacral nerves (and the midbrain + medulla). It is thought that by consciously stimulating the sacral nerves through the process of actively controlling the muscles at the base of the pelvis, that we also can influence the nervous system towards a more relaxed mode.
Since we tend to be stressed out people, the ability to access and stimulate the nerves that control body processes related to relaxation and pleasure seems pretty desirable and powerful. It offers the potential to choose to have relaxation, rather than fear rule our experience. And a relaxed mind is a clear, creative and happy mind!
Even though mulabandha sounds great so far (as long as it’s not too weird to think about contracting muscles in the pelvis) I’m not sure it’s a good idea to recommend it to everybody in a ordinary multi-level flow yoga class. (Ashtanga classes are an exception because mulabandha is intrinsic to that practice. But even Ashtanga yogis would benefit from detailed attention to what they are doing “down there”.)
To activate the pelvic floor muscles we do much better when we understand some of the basic anatomy of that area, and even then to find out the experiential aspects of exploring, understanding and appropriately using that anatomy can take some time.
Mulabandha is not a simple subject; it is a vast process that is best approached with reverence.