This mainly stretches your groin and upper inner-thigh area, but also makes some demands on your lower back. It is often called the butterfly stretch or frog stretch because of the shape that your legs make when you perform it.
Sit down with your back straight up (don't slouch, you may want to put your back against a wall) and bend your legs, putting the soles of your feet together. Try to get your heels as close to your groin as is comfortably possible. Now that you are in the proper position, you are ready to stretch. For the passive stretch, push your knees to the floor as far as you can (you may use your hands to assist but do not resist with the knees) and then hold them there. This can be hard on the knees so please be careful. Once you have attained this position, keep your knees where they are, and then exhale as you bend over, trying to get your chest as close to the floor as possible. Hold this stretch for about 20 seconds.
The isometric stretch is almost identical to the passive stretch, but before you bend over, place your hands on your ankles and your elbows in the crooks of your knees. As you bend over, use your elbows to "force" your knees closer to the floor while at the same time pushing "up" (away from the floor) with your thighs to resist against your arms. Once again, please be careful since this can place considerable strain on the knees.