Here's how John Scott describes the crucial stage.
"At the exact time that you roll on to the point of your shoulders, press firmly through your palms. keeping your legs energized and, synchronous with the push through your palms, shoot your legs backwards". John Scott Ashtanga YogaAddendum: Watching this I think where I'm going wrong is failing to tuck my chin in. Would give it another go this morning.......................but I have a stiff neck.