Double leg stretch is the secound exercise in the 5 SERIES . It challenges the lower abdominal muscles while working the hip flexors, sartorius, as well as the quads and vasti medial muscle which helps retract the patella. It also helps to strengthen your deep core stabilizers so that your lower back will be protected and stronger.
An image I like to give in class is to imagine a ruler between your bottom rib and your hip bone. Maintain that distance as you reach your arms & legs away- creating two long levers. The arms should reach by the sides of your cheeks "take your top hat off" instead of allowing the arms to reach behind the head and thereby losing the rib to hip connection.
It is also helpful to maintain your gaze at your knees or toes while staying in thoracic flexion, and keep your arms in your peripheral vision as you circle them to towards your ankles. This will help you stay abdominally contracted. Stay tuned for more deconstructions from the 5 SERIES!