There are 20 leg behind head poses in Ashtanga yoga. The first series has one: supta kurmasana. The second series has already 4 leg behind head poses. It becomes rather advanced in the forth series. To move into a tripod headstand with the leg behind the head is confusing. I tried it lately. That was it, I guess for a very long time.

I learned supta kurmasana when I practiced second series already. It's so much easier to learn first eka pada sirsasana (one leg behind the head) from the second series. Suddenly I was able to do supta kurmasana after having learned to take one leg behind my head. It became even comfortable with time.

These days I work on that lightness again. I struggle with that asanas. I know now how it feels when it feels good to be in such a pose. All the 20 asanas have some craziness. I'm looking forward to learning them. All.