Stretching the rectus femoris of the quadriceps femoris group. The rectus femoris is the only quadricep muscle that crosses the hip joint. It crosses it anteriorly with a vertical direction to its fibers, so it flexes the thigh at the hip joint and it anteriorly tilts the pelvis at the hip joint.
Stretching all hip flexors, including the rectus femoris of the quadriceps group, is extremely important for clients with lower crossed syndrome.
This video covers the attachments, actions, and motion patterns of the iliacus, explaining the biomechanics of how the iliacus functions.