I have been teaching the "simpler" version of this pose to my students and realize how much more easy it is than the parsvabakasana (side crow) and many other arm balances, though it looks so complicated. Most of my students pick it up on first attempt, or in the following week, after regular practice. Which just goes to show that in India at least many of these poses are not being taught simply because the teachers are lazy or not as excited about the hatha yoga aspect of the practice. Too much blah, without too much practice makes for a very boring practice.
However, this version is rather tough. It is an upward lift of the legs against gravity and needs very strong oblique muscles and core strength.
* Focus on the oblique muscles to lift up.
* Keeping the hands as straight as possible, when starting off (the lifting arm will bend a bit, to accommodate the legs).
* Keeping the legs -- the bottom leg -- straight and to the side.
It is a very tough pose and very exciting to practice. The engagement of the oblique muscles kicks into acupressure points that stimulate the nervous system positively, and hits what I prefer to call "joy" points!
* Needs a very strong basic astavakrasana.
* Brahmacharyasana and lolasasana as preparatory poses.
* A steady boat pose too.
HERE ARE A FEW VERSES FROM THIS SITE, from a translation of Astavakragita. If you try to grasp them, you will finally understand what yoga is about. However, I advise those who wish to read the Astavakragita to pick up the inexpensive translation/interpretation by Ramakrishna mission(Khar, Mumbai) It is best to read such treatises through several interpretations. The translations may not quite vault you into the stratosphere of thinking which is Indian philosophy:)
A sage said: "First of all I was averse to physical activity, then to lengthy speech, and finally to thinking itself. Trying to think the unthinkable is unnatural to thought."
56. Just as the performance of actions is due to ignorance, so their abandonment is also due to ignorance.
57. The inner freedom of having nothing is hard to achieve, because it requires living as one pleases, abandoning both renunciation and acquisition.
58. Recognising that in reality no action is ever committed, the sage lives as he pleases, just attending to what presents itself to be done.
59. No benefit or loss ever comes to you, consequently live as you please, abandoning the pleasant and unpleasant.
60. One person of pure intelligence may achieve the goal by the most casual of instructions, while another may seek knowledge all his life and still remain bewildered.