Brad's Iyengar Yoga Notebook
Dandasana is the foundation of all seated asanas -- forward bends and twists.   As with all seated poses, Dandasana can be made easier by placing height in the form of folded blankets under your buttocks.   This lessens the need for hamstring flexibility and allows you to sit in the pose more easily by decreasing the amount of flexion you must establish in your hips to keep your torso perpendicular to the floor.   However, even a beginner should practice Dandasana at least occasionally without any height under the buttocks.   One of the key lessons this asana teaches is the feeling of contact (Skt. sparsa) with the backs of the legs against the floor, the feeling of grounding the femurs into the floor, the feeling of pressing the backs of the knees into the floor, and the spreading of the calves on the floor.   These feelings are key in virtually all of the seated poses and are learned first in Dandasana.   However the backs of your legs will not be able to touch the floor if you take height under your buttocks.
So practice Dandasana with intelligence.   Use height under your buttocks if you truly need it but also spend time in the pose without the height, even if it feels uncomfortable for you and causes your low back to round, because you want to begin to ingrain the important sensation of making contact with the backs of your legs fully against the floor.
In Dandasana three things are most important:
(1) To press your thighs down, grounding them into the floor,
(2) To lift your spine and torso up as much as in Tadasana,
(3) To lengthen your arms fully.
All other actions in this pose are basically just to enhance these movements.
Actions of the torso, hips, and pelvis
As you sit in Dandasana, manually move your buttock flesh, sitting bones, and clothes back and out to the sides away from each other.   Notice it is not just the skin and flesh of your buttocks that you move with your hands.   Reach right up there into the crack of your butt and move even the bones to the sides and back.   You want to be sitting on the front and inner aspects of your sitting bones.   The sitting bones in Dandasana are like the feet in Tadasana.
From this foundation of support, draw your torso up as in Tadasana, raising your sternum toward the ceiling.   Draw your sternum up away from your pubic bone as much as possible to lengthen the front of your torso maximally.   Lift your sternum, your top ribs, and your collar bones.   Draw the front of your chest upward strongly.   As much as you lengthen the front of your torso, lift your side ribs upward.   As you raise the sides of your chest, lift the base of your lumbar spine and move it gently inward.   Keeping your kidney area full and broad, also take your dorsal (thoracic) spine in.
Actions of the legs and feet
As you begin to focus your attention on your legs and feet in this pose, begin by noticing that the outer edges of your feet tend to relax forward more than the inner edges of your feet when you just relax your legs or are being lazy.   This is true of the forward leg or legs in all seated poses.   Extend through the inner edges of your feet with your intelligence more than through the outer edges since this is what is required to take your feet forward evenly.   First extend more through your inner heels, and then press the balls or mounds of your feet evenly forward so that you incline your feet forward like stepping on the gas of your car.   Press more through the mounds of your big toes and draw the mounds of your little toes back slightly so that the balls of your feet are pressing forward evenly.   Now pull your toes back (while keeping the balls of your feet extending forward) and spread your toes gently to bring intelligence into them.   If you can, touch your great toes together.   Remember to apply this foot positioning in each seated pose, twist or forward bend, in whatever foot is forward.   There are also times when we want to keep our feet vertical -- perpendicular to the floor -- but the above description is one way to bring more intelligence into your feet.
In adjusting your feet, you will notice that you have naturally placed more intelligence in your legs as well.   Begin to work on your leg actions by moving the flesh on the backs of your thighs from inside out (to help your front thighs rotate inward) and move it toward your sitting bones as in Adho Mukha Svanasana.   You can move this flesh manually (with your hands) and then work your way down and do the same thing for your calves, spreading the flesh from the back midline of each calf out to the sides and also "milking" the flesh of your calves toward your heels.   The calf muscles move away from the knees and toward the heels in almost all seated poses in the leg or legs that are straight.
Join your inner knees and ankles.   Draw the tops of your thighs toward your groins.   Ground your femurs to the floor.   Press the fronts of your thighs down into the backs of your thighs.   Press your legs down to the floor strongly.   (Pressing your legs down is the same action as moving your legs back in the standing poses).   Make sure the head of your femur bones are descending to the floor.   Have the back of your thighs fully in contact with the floor.   Especially take your outer femur bones down toward the floor.   At the same time, internally rotate your thighs so that the centers of your thighs face directly upward toward the ceiling.   Turn your thigh muscles from outside inward so your legs are perpendicular to the floor.   Press your inner thighs downward to facilitate this action.   So, your inner thighs press down as your outer femurs press down.
Press not only your thighs down into the floor, but also press your kneecaps into your knee joints and press the backs of your knees into the floor to straighten your legs and open the back of your knees.   Since the backs of your legs are on the floor and your legs are not weight-bearing, you do not have to worry so much about hyperextension of your knees (unless you are sitting on a raised seat in which case pressing your knees down too strongly would hyperextend your knees).   Your heels and the backs of your knees both press into floor.
As you do these more mechanical actions with your legs, don't lose the overall feeling of the connectedness of the pose.   Feel that as much as you press your legs downward, you are raising your spine and torso upward.   Feel that as much as you press your outer femurs down toward the floor, you open your floating ribs out to the sides and also lift them upwards away from the floor.
Actions of the hands, arms, and shoulders
If you have long enough arms, you can place your hands on the floor to your sides and just behind your hips.   Some people have arms that are too short to do this and they need to use blocks to place their hands on.   In any event, your arms should stay straight in Dandasana.   Your fingers should be pointed forward -- resists the tendency we all have to point them back, even though it may feel that you have more leverage that way.
Rotate your upper arms outward to broaden your chest and collar bones.   Pull your elbows behind you and toward each other to open your chest more.   Press your hands down on the floor behind you to lift your torso as much as possible.   Press your shoulder blades into your body from back to front to open your chest and draw the tops of your shoulders back and downward.