I heard rather late about it. To come up from halasana to ubbaya padangusthasana shall happen with straight legs. This makes this sitting asana to a balancing pose.
The usual suspicions help:
1. Correct breathing: Come up with inhaling
2. Engage the bandhas.
3. Dristhi: look upwards to the toes and stretch the body.
4. What helps me is that I stretch the feet upwards. I can balance out the pose with these little movements of my feet.
A trick: I have learned this by M, my yoga teacher here. When you don't manage it to come up with straight legs, put a very soft blanket on your mat and try it on this blanket. Then the rolling becomes slow motion. It's easier to learn the timing (matching the breath with the movement) when coming up slower.
To learn coming up with straight legs with a soft blanket on the mat is especially helpful for the next pose urdhva mukha paschimottanasana.
What else: Tim Miller, yes, THE Tim Miller, the one who has his studio on Encititas, the one who was interviewed in the book about Guruji about his experiences with P. Jois, India, and the practice, has started a blog. Here is the link.
If you like, join me on facebook. The link is on the right side of this blog. Thank you.
All yogis/yoginis need that book.