The Couch Stretch targets the anterior aspect of the thigh and the hip flexors.
Stabilization of the core is extremely important in making this stretch functional. The hip flexors (iliopsoas) attach to the front side of the lumbar spine, and if the spine isn't stabilized, the low back will begin to sway.
Skip to the :53 mark here for a video demonstration.
Changes in trunk position are exactly what therapists and chiropractors are targeting to decrease when people report with back or hip pain.
Be sure to engage the abdominal wall to prevent arching the back.
Advanced trainees can add a hip-hinge movement once they master the basic stretch.