Ramaswami refers to the On one leg series as tapas (heat) or austerity postures.
My favourite austerity story is the tragic tale of Amba from the Mahabharata. Rejected by the great warrior Bima, the man she loves, Amba practices austerities, standing on one toe for twelve years in the snow. Siva grants her the boon (gift) of telling her that in he next life she will kill Bima.
One legged postures build strength in the legs, protecting the knees by strengthening the muscles above and below. However, while building up those muscles the knee is at risk in the one legged squats, especially coming back up so caution is advised. I've had operations on my knees in my youth so, while happy to squat down into these postures I still tend to come back up but putting down my hands and returning via uttanasana (forward bend) just to be on the safe side, especially in the colder weather, see video link.
Quietening the mind
No other postures seem to quieten the mind as much as balancing postures, if your having a sever attack of monkey mind one morning a One leg subroutine might be just the thing.
Balance can be improved by fixing the gaze on one point. In most postures in the On one leg sequence the chin is down as is the gaze.
Slowing and regulating the breath can help with balance as well, make your ujjayi a little stronger a little more forceful by tightening the throat, the glottis a little more.
Engaging the bandhas (see practice guidelines DAY 1), drawing up the anus and sucking in and up the belly but not too strongly which might send you off balance.
Many find this challenging, don't worry, we get to work on half and full lotus in many of the Vinyasa Krama sequences, taking different approaches. In the Asymmetric sequence we have a long build up to half and full lotus while seated. In Supine we approach the postures laying down and upside down as well as in the Inverted sequence. In the Lotus sequence several of the Asymmetric subroutines reappear as preparation for full lotus.
I was thinking this morning that standing on one leg is actually a good first approach to half lotus. The danger of lotus is thinking it's all about the knees, bending and twisting and pretzeling them into position. Lotus is actually all about the hip.
Look at the first picture, Bhagiratasana, we place the foot against the inner thigh. To do so we bend the knee, lifting the foot, then take the knee outwards bringing the foot to the inside of the thigh. The hip is a ball and socket joint, we've already begun to rotate the ball in the socket. To go deeper and into half lotus and Vrikmasana (tree pose), rotate the ball a little further in the socket, taking the knee a little further out and down which automatically brings the foot up towards the groin. Here's the bit I like best about approaching it standing, to go a little deeper still and bring the foot up a little higher into the groin we allow the knee to drop a little further. There's been no wrenching and pulling up to get the foot into position it's done almost of it's own accord and is worth remembering when we approach half lotus seated.
Of course depending on our flexibility we might knot get the foot as high up into the groin as we might wish, we may have to make do with half way up the thigh so we can support the foot with our hand. As we start to squat the bend in the knee will give us some support allowing us to let go and take the hand position, whether out in front or above the head.
Gazing down, engaging bandhas, focusing the breath all help. At first go just as far down as feels comfortable then come back up.
We lower on the exhale, come back up on the inhalation.
Ideally we will lower all the way and stay for a number of breaths, 3-6 but on first encountering squats it might be a good idea to go down as far as is comfortable on the exhale and then come straight back up on the inhale a few times, then if we feel confident go down a little way stay for a breath, perhaps two and then come back up. We might also try it holding onto the back of a chair.
As with the Lotus we will come across the arm behind the back while holding onto the toe bind in several of the sequences, seated, reclining and inverted approaches.
While working on the bind we might hold the raised right foot with our left hand and then reach around with our right and hold the inside of the left elbow.
To get in a little deeper we can walk our fingers down our forearm towards our foot, a little further each day.
Another approach is to stretch up out of the pelvis, twist to the right and take our shoulder over and back as if we were putting our hand through a sleeve. When we have gripped as far down our arm as we're able or perhaps our hip bone we then take a firm hold and then straighten back up and realign ourselves as best we can.
Yet another approach is to put our right arm behind our back, the back of our hand just above our hip and then turn and then twist to the right sliding our hand over our hip towards our foot.
It took me quite a while to get this bind, I seem to remember wrapping a belt around my foot and holding the ends, working a little further down the belt each day, each week.