Brad's Iyengar Yoga Notebook
Janu Sirsasana is a good pose to stretch the kidney area which renders a different effect than that of Pascimottanasana.   However, many of the instructions given under the section for Pascimottanasana are applicable to Janu Sirsasana as well.   Janu Sirsasana differs from Pascimottanasana in its asymmetry in the legs and hips, and in the twisting action this asana imparts to the spine.
As in almost all sitting forward bends, 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.
For seated forward bends, many people like to spread a blanket out over the sticky mat because these are not generally poses that you need to "stick" in.   The blanket will also reduce the pressure you may feel under your front heel or your rear ankle.
Begin Janu Sirsasana and all seated poses from a well-established Dandasana.   Draw the knee that will be your rear knee back using the strength of your hand pulling the inner knee back from Dandasana.   Do not use the muscles of your leg to pull your knee back.   Relax your leg totally, especially your groin on that side, as you pull your knee back.   As you draw the knee back, try to keep it on the floor, although if it comes one inch or so off the floor that is OK.   If your knee comes three or four inches off the floor, then don't take the knee so far back.   Place blankets under your rear knee to support it only if you have pain in that knee, otherwise just allow it to be off the floor a little bit if it is.
After drawing your rear knee back, if you are fairly new to this pose, you place the sole of your rear foot against your opposite (straight leg) inner thigh as in Vrksasana.   A more intermediate placement for the rear foot is with it drawn back a little further and the heel of that foot at the ipsilateral groin (at the very root of the thigh) with the sole of the foot turning upward toward the ceiling.   In other words, if you've drawn your right foot back, your right heel is beside your right groin fold.   You can allow your toes to touch your opposite inner thigh in this position.   This placement of the foot requires you to draw your rear knee back a little further forming a more obtuse angle of that leg with your front leg.   You may need to rotate your rear foot and shin bone with your hands as necessary to turn your foot upward facing.
Actions of the legs and feet
Turn your rear thigh outward and then lengthen out from your groin through that knee.   Draw your rear knee back as far as possible and press it downward to the floor.   The outer side of your rear shin should stay in contact with the floor as much as possible (use a blanket if needed for support).
Turn your forward (straight) leg slightly inward to ensure that your thigh is facing straight up toward the ceiling.   In all seated forward bends draw the entire leg that is forward toward the front of the room except for the quadriceps which pull toward the pelvis.   Pull your front kneecap in toward your groin and keep your front thigh contracted throughout the pose.   Lengthen the entire undersurface of your forward leg from the buttocks all the way through your foot towards the front of the room.   Next ground your forward heel and thigh.   Try to ground your forward leg to the point that there is no space between your leg and the floor (unless you are using blankets to sit on).   Press the back of your forward knee into the floor.   This action is important for the straight leg in all the seated forward bends.
First extend your forward heel away from you and elongate the Achilles area, then ground your heel and spread the toes of your forward foot and press forward through the ball of the foot, inclining your foot forward like stepping on the gas of your car, but pull your toes back while you continue to spread them.   Notice that the outer edge of the forward foot tends to stretch forward more than the inner edge when it is lazy, when you are not extending your consciousness there.   Extend through your inner foot edge more than your outer foot to take your foot forward evenly.   Also have the feeling of extending out more through the inner aspect of your forward leg than the outer aspect.
Preparations for Janu Sirsasana
After placing your legs for Janu Sirsasana, there are several preparatory exercises you can do to prepare your body for moving into the classical asana.   First try lowering your torso straight out to the side away from your bent leg, over onto your hands, then down onto your elbow, and even lying onto the floor if you are able to while still keeping your rear knee on the floor.   Spend some time there   Then sit back upright and re-establish your leg position.
Next, place the hand corresponding to your rear leg on the outside of your forward thigh and take your other hand back and place it on the floor behind your torso.   Use the strength of your hands to twist your torso away from your rear leg and spend some time working on this twist.   Then sit back upright and re-establish your leg position.
Another optional preparatory exercise is to take your torso forward midway between your two legs (like Upavistha Konasana except with one leg bent), so your torso is at a 45 degree angle to your front leg, and rest your torso on a bolster or blankets for about five minutes, bending over the foot at your groin.   In this position, keep both legs active while you pacify your torso and abdomen.
Another common preparation for Janu Sirsasana is to place your front-leg hand on the floor beside your hip and reach with your rear-leg hand to the outside edge of your forward foot.   You can use this moment to help center your torso over your forward leg as much as possible using the strength of your arms (pressing with your hand on the floor at your hip and pulling with your hand on your foot).
After doing any or all of these preparatory exercises, bring your torso back upright and spend some time re-establishing your leg actions as discussed previously.
Moving into Janu Sirsasana
Most of us are in too much of a hurry to take our torso down toward the forward leg in the seated forward bends.   It is much more important to take the torso up first, lifting it upward as much as possible.   Before taking your torso forward at all into Janu Sirsasana, draw your torso up into Tadasana torso, 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.   Pull the skin up the front of your torso to lengthen it.   Also stretch from your front heel all the way up the back of your forward leg to the buttock and then extend that stretch up the back of your body.   Draw your lower spine inward and upward.   Do this action in every seated forward bend.
It is common in the sitting forward bends to be able to open the chest well but to fail to open the area from the pubis to the navel and from the navel to the sternum.   You must get lift in your abdomen as well as your front ribs.   If you can't feel this lift, raise your seat more.
Most of us need to spend more time in this torso-upright position working on opening the front of our torso before taking the torso forward any amount into Janu Sirsasana.   When you do begin to take your torso forward, strive to maintain this elongation of the front of your torso.   Keep recharging the lengthening of the front of your body even after going down into the pose with your torso.
When you are ready to take your torso downward over your front leg, use a slow, progressive approach, taking these three steps over and over to come into the pose:
(1)   With your front-leg hand pressing on the floor just behind your hips, lift your torso upward more from your hips.
(2)   With your rear-leg hand on the outside edge of your forward leg, twist your torso more toward the front leg to center it over your leg.   You want to take your head in line with your forward leg and you try as much as possible to align your torso over that leg more and more with time.
(3)   Take your torso down toward your forward leg a little bit more.
Return to the first step and repeat these three steps over and over again until you are reach your maximum stretch for a given day.   Only after your have reached your maximum stretch in this way, do you then take both of your hands to your front foot.   If you are grasping your foot, have your thumbs on the same side of the foot as your fingers.   Use only a gentle pull with your hands on your foot to provide direction for the torso, not to overcome what stiffness you feel in your spine.   You want to lengthen your torso forward mainly with the strength of your torso and spine muscles, not with the pull of your arms.
More actions of the torso, hips, pelvis, and head
Strive to center your torso over your forward leg as you take it forward, though it will never quite make it perfectly.   You will need to draw the forward side of your torso (the side that is the same as your straight leg) backward and move the rear side of your torso forward to achieve this alignment.   Draw the ribs on the rear side of your torso both forward and toward the outside of your straight leg.   Also pull the outer aspect of your front hip back to help center your torso over your front leg.   Similarly, draw your rear hip forward toward the forward foot (even though your knee stays back as much as possible) to assist with your torso turning.   Try to establish your navel and the center of your chest (sternum) as closely over the center of your forward thigh as you can.   Try to make your torso symmetric over your front leg even though from the pelvis down you are asymmetrical.   To do this, your lumbar spine must twist away from your rear leg.
As you are taking your torso toward your leg, as in all forward bends, rotate your pelvis forward over your front thigh.   Focus your energy into moving your spine forward rather than downward toward your leg.   Don't forget the lengthening of your front torso.   Draw your sternum up away from your navel.   You want your front lower ribs to be "flying" out over your thigh and to land as far down your leg 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, or touch your "chin to your shin" down below your knee.   As you are going forward, keep the back of your head and neck aligned with your spine.   Resist the tendency to stretch your head toward your legs to try to look deeper in the pose.   That doesn't fool anyone; I've tried it.   Also, don't make the common mistake of letting your chin jut forward.
Feel the inner part of your front groin pull into your pelvis.   Draw your pubic bone back between your thighs and toward the floor.   Have the feeling of retracting your front leg into and under your pelvis as much as you are stretching your torso forward.   This action is similar to drawing your leg muscles upward toward your pelvis in the standing poses.   Take your rear hip bone toward your front foot and roll your rear buttock toward your front foot, but keep your rear sitting bone pressing down to the floor as much as possible.   Your rear buttock will become light on the floor, but keep your rear groin descending.   Keep your rear leg on the floor and that thigh turning back throughout the pose with its knee pressing downward -- the anchor of the pose.
When you are in a deep expression of the pose, with your torso near your leg, raise your front-leg kidney upward and take your rear-leg kidney downward and forward diagonally toward the outside edge of your forward foot.   Reach your rear side ribs toward the inner thigh of your forward leg.   Try to balance both of your kidneys moving forward symmetrically.   Adjust your kidneys so they are balanced, level, and even, though this is the hardest sitting forward bend to achieve that symmetry in.   Try to achieve the same balance with your lungs and shoulders moving forward.
Actions of the hands, arms, and shoulders
Widen your elbows out to the sides and lift them toward ceiling along with your upper arms and armpits to broaden your chest.   Dropping the elbows compresses the sternum.   Use your arms both to open your chest and to draw your spine up and out of your pelvis.   Take your dorsal (thoracic) spine into your body.   Draw your shoulder blades into your back to assist your chest in expanding and moving forward.