What is the best way to explain the action of Mula Bandha to somebody who is new to Ashtanga?

I just started teaching an Ashtanga class once a week at a local studio. For various reasons, I have decided to call it "Vinyasa from the Ground Up" rather than Ashtanga; but the class is very much Ashtanga through and through. Last night, for instance, three people showed up, and I brought the class through the Suryas and standing postures up to Prasarita Padottanasana A and B. I then ended the class with a modified finishing sequence (substituting Bridge for Urdhva Dhanurasana, having a shortened shoulderstand sequence, no headstand, and a simple crossed-leg seated posture in place of padmasana).

Although this is not a mysore class, I plan to teach as closely to mysore style as possible. Which means that I will introduce one or two new postures a class. I think this is a good way to go. It might get a bit challenging as new people arrive later on; maybe when I get to that point, I'll have to get these new people to stop at certain points in the sequence while the rest of the class move on with the postures they have learnt. Hmm... what will these new people do in the meantime? Watch while the "older" students do their postures? I don't know, I'm sure I'll think of something. If you have any suggestions, I'll love to hear them.

What I have done so far (last night was only my second class) is simply bring students through the postures on a purely postural level. I emphasized the breath, drishti and basic alignment, but I did not go into the finer points, like the bandhas. I just thought that somebody who is new to Ashtanga probably already has enough things to contend with, as it is. Perhaps I can talk about the bandhas when the students have come a few more times, and are more comfortable with the sequence.

Talking about the bandhas has always been an interesting topic for me. Different teachers use different descriptions when trying to tell students to engage the bandhas, especially mula bandha. And honestly, I have yet to find a completely satisfactory way of explaining the bandhas, especially mula bandha. Here are a few ways of explaining mula bandha that I have encountered so far:

(1) Squeeze/contract your anus: Guruji is reported to have explained the bandhas in this way. I do not think this is a very helpful explanation. For one, spoken with a certain South Indian accent, "Contract your anus!" is liable to be misheard as "Contact Uranus!" Which would leave the beginning Ashtanga student very bewildered, to say the least ("Wow, who knew that practicing this yoga involves getting in touch with extra-terrestrials?"). 

Joking aside, there are other more practical problems with this explanation. To begin with, strictly speaking, simply contracting the anal sphincter is actually ashwini mudra, not mula bandha. Secondly, anatomically, the action needed to activate mula bandha actually involves a subtle lifting of the perineal muscles; squeezing of the anal sphincter is at most a by-product of this action, and not the action itself.  Moreover, I get the sense that new students tend to overdo the squeezing of the anal sphincter: The result is that other surrounding muscles that do not need to be squeezed or contracted get contracted too, resulting in unnecessary muscular tension and tightness.   

(2) Do the same thing that you would do if you were trying to stop the flow of urine while peeing: This is probably a better way of explaining the bandhas. But--and maybe this is just my body--I have actually tried stopping the flow while urinating, and I am quite sure that while the muscles needed to stop the urine flow are used to engage mula bandha, the actual engagement of mula bandha requires a more subtle action than simply stopping the urine flow. This, at least, is what I feel from my own experience.  

(3) Engage Uddiyana Bandha, and Mula Bandha will tend to follow: Personally, this explanation actually works best for me. I'm not quite sure why, but there's something about gently drawing in that spot about two inches beneath the navel (i.e. engaging Uddiyana Bandha) that also tends to cause me to engage mula bandha almost without being conscious of it. Perhaps it's because I used to practice martial arts, and engaging uddiyana bandha has a lot in common with the actions that are needed in order to access the hara or dantien in Japanese and Chinese martial arts, respectively. Because of this, uddiyana bandha has always been more readily accessible to me than mula bandha. This simultaneous engagement of uddiyana and mula bandhas is most pronounced when one tries to jump back. In order to even try to lift off the mat into Lolasana, one has to engage Uddiyana Bandha (even if one has no idea that one is doing this). This engagement of uddiyana bandha, in turn, also almost automatically causes one to engage mula bandha. Try it; you'll see what I'm saying.   

(4) Do the postures regularly, and you will find yourself naturally engaging the bandhas with time: David Williams is a proponent of this naturalistic approach towards the breath and bandhas. A few years ago, at a workshop, somebody asked him how to breathe during practice. Instead of giving a technical explanation (closing the glottis, Darth Vader sound, etc, etc.), Williams simply said that all one has to do is to keep breathing, and everything else will just fall into place. If I remember correctly, he gave the same response when asked about the bandhas as well: Just breath evenly throughout the practice, and bandha-engagement will come naturally.

These, at any rate, are the explanations of mula bandha that I know about. If you have other explanations, I'll love to hear them. I'm still undecided as to which one I will use when it comes time to explain mula bandha to the students in my class. Maybe I'll use (3), since that is the one that appeals most to me personally. But we'll see.