This skill requires dramatic abdominal and hip flexor strength. Tight hamstrings and lower back will make this significantly more difficult.
Using the parallel handles jump to a support position. Work hard to retract your shoulders and press down hard into the handles to keep your upper back from rounding out. Lift your knees to your chest and then extend one leg out in front. Slowly return the leg before extending the next. If you find yourself falling backwards as you extend your legs it is most likely due to tight hamstrings or low back forcing you to lean back in order to elevate your legs. However, it could be due to weak hip flexers.