One more thing that I remembered practicing this earlier, I think we tend to be afraid of falling out of this and landing awkwardly and painfully with our leg still behind our head. I've stumbled in this a couple of times, this evening included and always the leg pops back from behind the leg neatly giving you time to stop yourself falling. It's tricky, can take a couple of attempts if you haven't practiced it for a while or if it's your first time.
NB: These are practice notes that will be tidied up and put into the new edition of my Vinyasa Yoga Practice Book along with the current sequences and subroutines. The book can be freely downloadedHERE. There is a page on Facebook HERE with all the latest sheets and updates. This book is in no way a substitute for Ramaswami's Complete Book of Vinyasa Yoga.