I just saw the above video, in which David Garrigues explains the mechanics of mula bandha in very clear and succinct terms, combining an intimate knowledge of pelvic anatomy with the energetics and mechanics of engaging mula bandha. Definitely a pleasure to watch and listen to. I have a couple of thoughts on his explanation:
(1) Early in the video, David gives us an overview of the structure of the pelvic area. He explains that what we commonly think of as the pelvic muscles is actually a diamond-shaped group of muscles that are bounded by the sit bones, the pubic bone and the coccyx.
Within this diamond, we can further isolate three groups of muscles. Firstly, the back part of the diamond consists of the anal muscles, which we engage by contracting the anus and engaging Ashwini Mudra. Secondly, the front part of the diamond consists of the urethra muscles, which we isolate by engaging Badroli Mudra (I hope I'm spelling this correctly; I've actually never heard of this mudra before, so I'm transliterating as best as I can by listening to David's explanation). David doesn't go into too much detail about this mudra, but I suspect that we engage this mudra by engaging the muscles that we engage when stopping the flow of urine while urinating.
Thirdly, there is a tendon that separates the anal and the urethra muscles, or the front and the back parts of the diamond, if you will. This tendon is the transverse perineal tendon. To engage mula bandha is to isolate and draw up this tendon, thereby creating the lifting action that we associate with this bandha.
David remarks that isolating this one tendon takes a lot of work over a period of time. As I understand it, he recommends starting with contracting the anus and engaging the urethra muscles in practice; over time, the practitioner will get a finer sense of where exactly mula bandha is.
I'm not sure I really agree with all of this. I'm no expert in mula bandha, but in my own practice experience so far, contracting the anal muscles seems to be a rather counterproductive action. It tends to create unnecessary tension in the glutes, which leads to unnecessary tension in the lower body. At least, that is my experience thus far. Hmm... maybe I should get in touch with David himself and seek more clarification on this. Anybody have any thoughts on this?
As for engaging Badroli mudra, I think this might be a bit more helpful, at least for me. At any rate, I often find myself engaging the muscles in the front of the pelvis almost without realizing it during practice, and it seems to be working fine for me thus far. As for drawing up the transverse perineal tendon... well, if I can say that I am definitely drawing up on this tendon every single time that I practice (or even every moment of the day, as Sharath would say), then I should be an Ashtanga Rock Star by now, right? :-)
(2) Later in the video, David speaks of the feet (pada), legs and the pelvic region forming what he calls a gross mula bandha. The general idea is that by working on rooting down through the feet and lifting the arches in the Suryas and the standing postures, we get a bigger canvas on which to understand the general workings of mula bandha, and are, in turn, able to gradually apply the insights gained from this bigger canvas to activating the "actual" mula bandha. I really like this explanation. It fits in with the general objective of Ashtanga yoga and indeed, all of hatha yoga, which is to work from the gross to the subtle levels of being.