This stretch is very good for working toward a side (chinese) split (see section The Side Split). This exercise should be performed after you have stretched each of these areas individually with prior stretches (like the ones mentioned above).
Start by lying down with your back flat on the ground and your legs straight together in the air at a 90 degree angle. Try to have your legs turned out so that your knees are facing the side walls more than they are facing your head. Slowly bring your legs down to the sides, keeping your legs straight and turned out. When you reach the point where you cannot bring them down any further into this "lying" side split position, leave them there.
Now for the stretch: With your feet both flexed or both pointed (your choice) use your arms to reach in and grab your legs. Each arm should grab the leg on the same side. Try to get a hold of the leg between the ankle and the knee (right at the beginning portion of the calf that is closest to the ankle is almost perfect). Now, exhale and use your arms to gently but steadily force your legs down further and wider (keeping the legs straight) getting closer to the lying side-split position (where, ideally, your kneecaps would be "kissing" the floor). Hold this position and keep applying steady pressure with your arms for about 20 seconds.
For the isometric stretch, you do the same thing as the passive stretch except that, as you use your arms to force your legs wider, use your inner and outer thigh muscles to try and force your legs back up together and straight (like a scissors closing), but apply enough resistance with your arms so that no motion takes place (this can be tough since your legs are usually stronger than your arms). You may find that you get a much better stretch if you use a partner (rather than your own arms) to apply the necessary resistance.