Brad's Iyengar Yoga Notebook
Halasana is the one of the first inversions practiced after Adho Mukha Svanasana (along with Prasarita Padottanasana).   This pose and its neck action has a quieting effect like Sarvangasana and Setu Bandha Sarvangasana and is good for restorative work.   These poses introduce and develop Jalandhara Bandha.
The usual way to enter Halasana is to take your feet down to the floor from Sarvangasana.   If you choose to use several blankets under your shoulders for Halasana, or if you are an experienced practitioner and have good flexibility, you can also move into Halasana directly from standing by placing your hands on the floor (into Adho Mukha Svanasana), tucking your chin into your chest strongly, walking your feet forward toward your hands, and bending your elbows to set your shoulders down into Halasana.   If you move into Halasana in this way, be sure your head does not touch the floor on the way down.   You can also come out of Halasana by reversing this procedure, though the typical method is to roll out of Halasana by taking your back and then your legs to the floor.
If you cannot reach the floor with your feet from Sarvangasana, make sure you have a chair or other support to take your feet down onto and just do Ardha Halasana until you have enough hamstring and hip flexibility to take your feet to the floor.   It is especially important to remember that hanging in half Halasana without any foot support is not good for your neck.   You might have a tendency to do that, for example, when putting your belt on for Sarvangasana with the idea that you're just going to be in that position for a moment.   Resist that tendency.   Either take your feet all the way to the floor for full Halasana or use a chair or other support and do Ardha Halasana.
Also if your low back rounds a lot in Halasana, you need to use a chair for your feet and do Ardha Halasana until your hamstrings lengthen enough to allow you to take your feet to the floor without compromising the length of your spine.   It's more important to have your back and legs straight than it is to have your toes on the floor.
Actions of the torso, hips, and pelvis
When coming into Halasana from Sarvangasana, keep the front of your spine as long as possible as you lower your feet to the floor.
In the pose, lift your sitting bones toward the ceiling maximally as in Adho Mukha Svanasana.   Move your pubic bone away from your sternum as much as possible to lengthen the front of your torso.   Try to keep your torso perpendicular to the floor as much as possible.   The idea in Halasana is to elongate your spine vertically as much as possible, stretching up through your sitting bones.   Unlike Sarvangasana, your hips in Halasana are not directly over your shoulders.   Instead, they move to a position approximately over your chin, which places a little more stretch in the back of your neck.
As in all forward bends, you want to bend from the front of your hips (your groins), not from your low back.   Although in this pose it is acceptable to have a slight curve in your lumbar region, lift your hips as much as possible to attempt to keep your low back from rounding.   Deepen the fold at your groins.   These principles apply to almost all forward bends.   However, in Halasana, do allow your sitting bones to move toward your heels and feel how that allows your abdominal organs to remain soft.   Try not to hold tension in your abdomen or groins.
As in Sarvangasana, expand your sternum vertically and horizontally to open your chest, though it is difficult in this position.   Observe the space between your lower ribs and your thighs and open that space by lifting your pelvis, groins, and the root of your thighs upward.
Actions of the legs and feet
When first practicing Halasana, keep your feet about one foot apart to help open the backs of your legs more.   As you gain experience in this way, you can bring your feet together.   Engage the skin of your front thighs strongly into your quadriceps and your quadriceps into your femur bones to lift your thighs through the back of your legs toward the ceiling as much as possible.   Draw your kneecaps in toward your groins.   Feel the back of your knees stretch strongly and the backs of your legs open fully.   Extend your heels away from your legs.
Keep your legs inwardly rotating.   Open your eyes and look at your thighs to make sure they are directly facing the floor and push them away from your face toward the ceiling.   Take your inner thighs and inner knees higher and higher.
Keep your feet vertical.   Stand on the tips of your toes rather than the balls of your feet.
As your flexibility improves, you can try walking your feet toward your head while still keeping your spine long.   Then try walking your feet back away from your head as much as possible.   Then walk them back to a neutral position.
Actions of the hands, arms, and shoulders
As in Sarvangasana, roll your shoulders deeply underneath to be on the very tops of your shoulders.   If you have your hands on your back, have your hands as close to your shoulder blades as possible and your thumbs as close down to your armpits as possible, just as in Sarvangasana.   Lift your shoulder blades toward your rear pelvis and draw them into your back to help to open your chest.
Every few breaths, adjust your hands down a little and lift up through your legs a little more.   Your base (shoulders, arms, and elbows) moves down and everything else lifts upward.
One good way to work with this pose is to have a partner take a belt under the very tops of your thighs (close to your groins) and lift the belt strongly toward the ceiling.   Then try to imitate that feeling in your own pose.
One variation of Halasana (which is perhaps the more classical form of the pose), is Halasana II.   Here, you take your hands off your back and clasp them together with your fingers interlaced, and take your arms fully straight down onto your blankets or the floor.   You want to extend your arms strongly away from your armpits in this position.
First try to get your arms to be absolutely straight with your elbows pressing into the blankets and then develop the ability to interlace your fingers while still keeping your arms perfectly straight.   If you can straighten your elbows but you cannot keep them straight when you interlace your fingers, hold a strap or rope between your hands and try to get your hands closer together with time by walking your hands together along the strap.
When you are able to clasp your hands and take your arms completely straight along the blankets or floor, then try to rotate your arms inwardly (pronating them) so your thumbs turn toward your back and then downward toward the floor so that you turn your palms face away from your back.   This is the same arm position as Sarvangasana II.
A more difficult variation of this arm position is to try to rotate your arms in the other direction (supinating them), taking your thumbs over the top and away from your back.   This variation has the advantage of turning your shoulders underneath more deeply.   In any of these straight arm variations, you can have someone place a bolster on your arms to help press them down more flat onto the blankets or floor.
Exhale slowly to roll out of this posture.   The way to roll out is to take your arms on the floor over your head and roll out slowly while continuing to look back with your eyes, keeping the back of your head on the floor.   Try not to let your head come off the floor when rolling out.   Roll out slowly.   Sometimes when rolling out of Halasana, a good exercise is to try to roll down one vertebra at a time, keeping your feet on the floor.   When you must raise your feet off the floor, do so, but continue to roll down one vertebra at a time and keep your shins as close to your face as possible on the way down.