BALANCE. Utthita Padangusthasana II or Extended Hand to Big Toe Pose. Very literal translation on this one. And it's hard. I'm not gonna lie, it's also one of my most favorite poses....but I love to balance on one leg, so there you go.
When I starting to regularly practice Ashtanga in Edinburgh, I got fixated on this pose and its point in the Primary series. I wanted to do it without any balance checks and thought it would never happen...and as usual, I was proven wrong. I do have some tips (other than just practice, practice, practice) for getting a solid version of this pose.
1. The lifted leg does not have to be straight to achieve success! Tight hamstrings are real.
3. Externally rotate your hip open before taking it over to the side.
4. Actively draw your low belly (uddiyana bandha) in throughout the pose.
5. Relax your face and smile...move your gaze very last.
Here's a great tutorial: https://www.youtube.com/watch?feature=player_detailpage&v=bY3qvUuBXK8