Brad's Iyengar Yoga Notebook
The least important part of Pascimottanasana is how deeply forward you can bend.   The most important part of this pose is how deeply you can surrender and relax into the forward bend.   Even the deepest forward bend should be cooling.   If you're doing Pascimottanasana and you're grunting away to pull yourself into position, then it's not Pascimottanasana.   You might be bending forward, but you're not forward bending.   This asana is not so much a stretch as it is a release.
This pose can be hazardous for the lumbar discs so it is important not to bend forward with a rounded low back.   Keep your low back concave as much as possible and your sternum uplifting strongly to maintain the extension of your spine and the length of the front of your torso, which is important in almost all asanas.   Pay attention to your hips at the angle between your legs and your lower abdomen (your groins).   This is the place to bend from, not your lower back.   Of course, the admonition to keep the low back concave and chest expanded in all poses is a guideline.   In a deep forward bend, you must round your low back somewhat just as a result of human anatomy.
As in almost all sitting poses, placing folded blankets or a sandbag under your buttocks will decrease the amount of forward bend you need in your hips to keep your torso upright and will make the pose more effortless.   This is especially useful if your hamstrings and hip flexors are very tight.   Make your seat as high as necessary to allow you to sit without tension in your low back and to allow your low back to concave naturally.   Using one folded blanket is a good idea even if you're fairly flexible.
To get the feeling of rotating your pelvis forward rather than just bending forward from your lower back, extend your legs straight out to the front, ground your heels and keep them in that position while you slowly bend your knees and allow the traction on the floor to pull your pelvis forward.   Just keep your torso upright as you practice this movement.   Try this once or twice before moving on to do the asana to get the feeling of tilting your pelvis forward.
As you sit in Dandasana to prepare for Pascimottanasana, make the adjustments that will make moving into the forward bend easier.   Turn your thigh muscles inward so your legs are straight up perpendicular to the floor.   Extend outward through the inner edges of your feet more than the outer edges.   Draw your buttock flesh and bones diagonally back and out to the sides on each side, one at a time.   Press the backs of your knees into the floor (unless you are sitting on a raised seat in which case this would hyperextend your knees).   Draw your torso up into Tadasana, raising your sternum toward the ceiling and taking your shoulders back to open your chest.   Draw your sternum up away from your pubic bone as much as possible to lengthen the front of your torso maximally, probably the most important action in any forward bend.
Actions of the torso, hips, and pelvis
As you sit in Dandasana, be sitting on the front of your sitting bones.   Manually move your buttock flesh, sitting bones, and clothes back and out to the sides to aid in tilting your pelvis forward.
The first goal of this asana is to take your spine up and make your legs do the work of the pose.   Only then, if you have the hamstring and hip flexor flexibility, should you move forward toward your legs and down with your torso.
Lengthen the front of your body before going down with the torso.   Pull the skin up the front of your torso to lengthen it.
As you begin to take your torso toward your legs, as in all forward bends, focus your energy into moving your spine forward rather than downward toward your legs.   You want to have the sensation that your front lower ribs are "flying" out over your thighs and you want them to land as far down your legs as possible.   First you want to touch your belly to your thighs, then your anterior ribs, then your chest to your legs, and then lastly take your forehead to your legs.   Do not try to take your head directly down to your legs.   Forget about that.   That is the last part of the pose.
As you achieve some depth to the pose, reach with your chest toward your feet.   The way to get flat is to get long.   Lengthen from your outer hips up through your armpits.   Do not allow constriction around your floating ribs -- move them forward along your legs.   Continue recharging the lengthening of your front torso after going down toward your legs.   This is the critical action of Pascimottanasana.
As in all forward bends, feel the inner parts of your groins pull into your pelvis.   Draw your pubic bone back between your thighs and down toward the floor.
Actions of the legs and feet
Though the classical pose is done with your legs together, if your hamstrings are tight, keep your legs about hip width apart for a while.
Roll the tops of your thighs inward to verify they are straight upward.   Be on the center of the back of each heel.   Extend your heels away from you.
Broaden the backs of your legs and ground the backs of your legs so they contact the floor maximally.   Try to ground your legs to the point that there is no space between them and the floor (unless you are using blankets to sit on).   If your hamstrings are tight, you can modify the pose by bending your knees slightly as needed to allow you to bend forward somewhat with a fully extended torso without rounding your back.   This is more important than having your knees straight.
You should extend through your inner and outer feet evenly, although achieving this action often means having the feeling that you are lengthening your inner legs out more through your inner heels and feet than through their outer aspect.   This advice applies to many asanas.   Pressing through your inner legs as you are rotating your thighs inward to keep them straight up will give you the feeling of spiraling your legs toward each other.
Move the backs of your thighs toward your buttocks and also draw the skin and flesh of your thighs in toward your pelvis.   Pull your kneecaps in toward your groins.   Keep your thighs contracted throughout the pose.   Have the feeling of retracting your legs into and under your pelvis as much as you are stretching your torso forward.   This has the same effect as drawing your leg muscles up toward your pelvis in the standing poses.   Contrast this with your shin and calf muscles which should be stretching forward toward your feet in this pose.
To achieve the proper extension in your feet, first lengthen your back legs to extend your heels away from you and elongate the Achilles area, then ground your heels, spread your toes and the balls of your feet (so that the skin on the soles of your feet is taught), and press through the balls of your feet so that they move forward more than your heels.   Anytime your feet are not on the floor supporting your body, you should give the balls of your feet some forward intention (or "upward" intention in the inversions), like stepping on the gas of your car, as a means of elongating your legs.   Take care not to overdo this action and compress your Achilles area.
Extend through both sides of each heel and both sides of each ball of your feet.   Lengthen your toes outward from the balls of the feet (not curling them downward) and spread them as much as possible.   Though you are pressing forward through your heels and the balls of your feet, draw your ankles and the arches of your feet back as in Tadasana.
Your heels and the backs of your knees both press into floor.
Actions of the hands, arms, and shoulders
When you take your hands to your toes, feet, or finally down to grasp your wrist around your feet, hold your feet in order to allow you to release tension in your back, not so much to pull your torso down or to stretch your legs.
Widen your elbows out to the sides and lift them toward ceiling to broaden your chest.   Use your arms to open your chest and to draw your spine up and out of your pelvis.
Draw your shoulder blades into your back to assist your chest in moving forward.   Resist the tendency for the shoulders to rise toward your ears.   As always, keep your shoulders down and away from your ears.
If your hamstrings are tight, allow your arms and forehead to rest forward onto the seat of a chair.   Remember to relax your head and neck.
One way to work on Pascimottanasana is to begin in Halasana.   Grasp your ankles and slowly lower your back to the floor beginning at your neck and going down one vertebra at a time while keeping your feet on the floor.   This will gradually take you into an upside-down Pascimottanasana.
After any seated forward bend (or a series of them), do a counter-pose to release any tension in your low back.   Good choices for counter-poses are Child's pose or lying on your back and bringing your knees into your chest one at a time, and then both together.   After you do this supine knees-to-chest pose, you can take your bent knees together to the floor on either side of your torso, then back up to your chest, and finally place your feet on the floor close to your buttocks with your knees bent toward the ceiling, hands on your chest, and relax for a few minutes with your low back supported by the floor (which is the point of bending your knees up since if you extended your legs straight out on the floor as in Savasana, there would be a slight arch in your lower back).
These counter-poses are also useful after a series of backbends.