Brad's Iyengar Yoga Notebook
Supta Padangusthasana is an excellent way to develop flexibility in your hamstring muscles and the back of your legs without placing strain on your back since your back is well-supported and kept straight by the floor.   This asana is ingenious in that it almost takes your low back completely out of the picture, allowing you to place tremendous stretch into your hamstrings without the danger of hurting your lumbar spine.   This is one asana that you must master one day or another since its mastery relates to all the sitting asanas as well as many other asanas.   It is part of the basic yoga vocabulary.   If you can do the basic poses with clarity, then the more advanced ones will come more easily.
Though the classical version of this asana has you grasping your great toe with your fingers, most practitioners will initially need to use a strap or belt around the upper foot.   Use only as much belt as you need to keep your upper leg straight.   The arm you use to hold the belt (the same side arm as your upraised leg) should also be kept straight in this pose.   Anytime you're using a strap around your feet (for instance in the forward bends as well), the most correct position for the strap is around the balls of your feet, although it is also fine to have the belt around the arches of your feet.
One way some people prefer to hold their strap in this pose is to have no loop actually laced in the strap but to have just one throw tied in the strap (holding it in your hand) with the loop around the arch of your foot.   This way allows you to let the loop slip a little larger by releasing your grip on the tied part, and you can tighten it just as conveniently by pulling on each loose end with your hands.
Before you raise a leg into the pose, lie on your back and grasp the sides of your mat to make sure you are well-centered on your mat.   Draw your buttocks away from your spine while your knees are still bent, then straighten your legs into Supta Tadasana to prepare for the pose.
Actions of the legs and feet
Your bottom leg is just as important as your raised leg in this pose.   Establish Tadasana in each leg.   The leg that remains on the floor should be kept firm, straight, long, and flat on the floor.   One way of learning to strengthen this action is to place the sole of your lower foot against a wall and press it into the wall for added leverage while doing the pose.   It also feels good if you can occasionally place a sandbag on your lower thigh or have someone press it down to the floor for you.   At the very least, keep the calf of your lower leg firmly on the floor.   Strive to do this with your lower thigh as well, though you may not be successful at it.
Your lower leg will have a tendency to turn outward slightly.   Resist this tendency by keeping it rotated inward so that your entire leg (including your thigh) remains straight up and your lower foot remains perpendicular to the floor.   Be on the center of the heel of your foot on the floor, or a little to its inner edge.   Turn your inner knee downward toward the floor to help this action and extend outward more through the inner thigh, leg, and heel of your lower leg than through the outer aspect.   This leg action also helps you keep your lower back slightly off the floor as it should be.   Don't be so focused on pulling your upper leg toward your torso that you neglect the Tadasana of your lower leg.
Keep engaging your thigh muscles into your thigh bones in both legs.   Bury the tops of both of your kneecaps to ensure your legs are poker straight.   Press your lower quadriceps into your femur, into your hamstrings, and into the floor.   Keep both buttocks and hips resting on the floor.   When you raise your upper leg up, resist the tendency for the raised leg to pull its buttock upward off the floor.   Pay attention to extending the back of your upper leg downward into the floor as well as upward toward the ceiling.   When the upper thigh is working, it tends to come toward the abdomen unconsciously.   Draw your raised thigh away from your abdomen, down toward your lower foot, in order to lengthen your torso.
The straightness of your lower leg is more of a bone action, whereas your upper leg involves more muscular actions.   Especially in the upper leg, pull your outer thigh muscles down from your knee all the way toward the floor into your ipsilateral hip.
Feel the stretch in the hamstrings of your upraised leg, not the back of your knee.   Take care not to hyperextend your upper knee -- keep the skin of your calf pressing forward deep into your shinbone as in Tadasana.   Your raised leg should be a "Tadasana" or an "Uttanasana" type leg.
Press both sides of your upper foot evenly upward, not allowing any twisting from side to side at the ankle.   Your upper leg does not come over to the midline, but stays centered over its hip with your outer ankle in line with your outer thigh and hip.
Spread your toes and extend through the balls of both feet as if lightly pressing on the gas pedal of your car.
Actions of the torso, hips, and pelvis
You must level your hips in this pose by taking your raised leg hip, groin, and side of hip away from your shoulder on that because it tends to hike up when you pull your leg up and start drawing your foot toward yourself.   You almost cannot overdo this action.   Almost everyone needs to do it more.   While your upper foot moves toward you, the head of the same side (ipsilateral) femur must move away (in the direction of your foot on the floor).   It is like a seesaw action.   While you do this, though, keep your groins soft.   To help you, you can have a partner take a rope around your raised thigh and pull it away from you to help you get a feeling for this action.
Although both legs and hips are very active in this pose, you are essentially at rest from the waist up.
It is essential to the asana that you keep your entire back, including both shoulder blades, on the floor.   Do not let the arm that is holding your upper leg be pulled up from the shoulder by the strength of the leg.   Keep that shoulder pulling strongly down toward the floor.   Another way of saying this is to keep both of your collar bones broad and extending to the sides.
Be honest in this pose.   Even though you may be able to take less belt by letting your shoulder lift off the floor, resist that temptation.   Seek to find Tadasana in your torso.   Lift your chest while at the same time drawing your shoulders to the floor.   Your lower back will arch naturally a little bit.   Lengthen your spine evenly on both sides from your tailbone up through the crown of your head.   Keep the back of your neck long, as in all poses.   One way of augmenting the lengthening of your spine is to reach over your head occasionally with your lower arm (the one that is not holding your upper foot or strap), extending it strongly along the floor to help remind you to lengthen your spine in that direction.
Otherwise, classically, place your unused hand on your lower thigh.
When you perform Supta Padangusthasana II and take your leg to the side, you will need less belt in this position and may even be able to hold your great toe with your fingers if you could not in the initial pose.   It is often good to have a rolled blanket flushed up against the side of your pelvis on the side that your leg is going down toward to help you maintain a level pelvis.   Do not lead with your toes toward the floor, but have your foot come down parallel to the floor and reach up with it toward your ipsilateral shoulder.   Draw your outer thigh into your hip and rotate the thigh of your side leg strongly toward your foot that remains on the floor.   Roll your navel toward the straight leg side (away from your leg that is lowering to the side) to keep your back flat on the floor.   Pay particular attention that the hip of your leg that is lowered to the side does not hike up toward your ipsilateral armpit.   Draw that hip down strongly toward your foot on the floor to lengthen the side of your torso.