The hip flexor muscle is actually two muscles the Psoas and Iliacus. The Psoas portion of the hip flexor attaches to the transverse processes of the lumbar vertebrae. Thus, the nursery rhyme is correct, the hip bone is connected to the spine bone!
To stretch the entire length of the hip flexor it is important to extend the hip joint of the rear leg, posterior tilt the pelvis of the rear leg, maintain the low back in neutral spine, bend the rear leg's knee and side bend the torso away from the rear leg. The hip flexor of the rear leg will be getting the stretch.
The Extended Side Angle Pose from yoga is a great stretch for the hip flexor. However, in this pose the rear leg's knee is maintained straight, decreasing the stretch on the hip flexor. In the video I am using the Hooked on Pilates MINIMAX to get a deep and effective stretch through the entire length of the hip flexor.