This stretches the adductors and also the hip socket.
Put your client's foot opposite their other knee and press their knee away from you. Place your foot or a rolled up towel under your client's hip, then bring your client's knee back toward you gently stretching the adductors. Work into the adductors and gently stretch your client's knee toward the floor.
Do this only with clients with healthy hip joints. Do not do this with clients with risk factors for osteoporosis, or clients who have had hip replacement surgery.