Halasana (Plough Pose) is an extension of Sarvangasana (Shoulderstand) and offers many of the same benefits. This posture lengthens the hamstrings and stretches the extensors of the spine, primarily along the thoracic and cervical vertebrae. For this reason, Halasana is effectively practiced as a counter-posture to backbending asanas or as a preparation for Sarvangasana.
To enter the posture, lie on your back with legs and arms straight, palms flat on the mat. With an inhalation, swing your straight legs up and over as you push into the hands and roll the shoulders back. If your toes reach the floor, point the feet and interlace the fingers. Keep the feet light against the floor using the back and bandhas to support the weight of the legs and actively press the clasped hands into the mat. If the feet do not reach the floor due to shortness of the hamstrings, take the hands to the back as in Sarvangasana to support the spine.
Roll the shoulders away from the ears to stabilize the scapula against the upper back. Lift the sternum toward your chin and ensure that the back of the neck is not flat against the floor. There should be space enough to slide two fingers beneath the neck to avoid hyper-flexion of the cervical spine. (Notice the light peaking through in the demonstration photo.)
Once the back and hamstrings have been comfortably lengthened, the primary challenge of this asana is the breath. It can be difficult to breathe freely with the intra-abdominal pressure this posture creates, particularly with the bandhas engaged. If you have trouble breathing in Halasana, remember to keep the throat soft and actively spread the ribs to make space for the breath. Gaze to the navel. Stay for 8-10 breaths or more.