Hi I believe there is a step missing on this video, in accordance to the book, the legs comes back to pose 3 (book) before it rolls back to the mat. If I am wrong I apologize.
Thank you for your question! Once you are in the Jack Knife position (arms stretched out, neck long, hips pushing up while feet stretch upward toward the sky), then the emphasis on coming down should be on the articulation of the spine - NOT the legs falling back to the third pose. Yes, the legs fall back - but they should be working on stretching upward while following your spine down through the center of your body one vertabra at a time. The control of the roll down and openness of your chest is extremely important in protecting your neck while in this exercise. So, even though the legs may fall back some, no farther than sixty degrees, they should not be the focus coming down, but rather the articulation and control of your spine. This is the 'Art of Control'. I hope this answers your question. Please let me know if you have any more questions.