This exercise is best done with the back flat on a table plus the head and shoulders rolled forward and the hands flat on the table besides the hips.
Next step, cross your legs, as in the frog sit-up, and roll the hips and crossed legs back towards the chest. This flexibility of the hips and lower back takes practice. This movement also requires that you suck in your abdomen as you perform it.
(IronMan Magazine Nov 1973 Vol. 33 No. 1 and Musclemag Vol 2 issue 1)