The explanation and the drawing of this pose is by myself, as I understand it - a PT and an Iyengar Yoga practitioner perspective.
1. Ropes help reduce gravity action on the body weight. Muscle action to keep the body stable and to elongate the trunk as well as the back legs is concentric and depends on amount of effort one generates.
2. Trunk flexion occurs as the Lumbopelvic region which acts as one unit while flexion takes place at hip region. The angle at the hip is around 90 degrees if there is muscles and other soft tissue flexibility.
3. and 4. Creating resistance at the heels and the finger allows the body to create elongation the right direction.
5. Support to the head allows one to benefit even more from the elongation in the right direction.