From Yoga Journal:
The pose described immediately below is a simplified variation of the full pose. Then we describe the full pose afterward.
Step by Step
Kneel on the floor. Stretch your right leg out to the right and press the foot to the floor. Keep your left knee directly below your left hip (so the thigh is perpendicular to the floor), and align your right heel with the left knee. Turn your pelvis slightly to the right (so the left hip point comes forward of the right), but turn your upper torso back to the left. Point the kneecap toward the ceiling, which will require you to turn your right leg out.
As you inhale, bring your arms out to your sides, parallel to the floor, palms down. Bend to the right over the plane of the right leg and lay your right hand down on the shin, ankle, or the floor outside the right leg. Contract the right side of the torso and stretch the left. Place your left hand on the outer left hip and push the pelvis down toward the floor. Then slip the hand up to the lower left ribs and lift them toward the shoulder, creating space in the left waist.
With an inhalation, sweep the left arm over the back of the left ear. The side bend tends to drop the torso toward the floor. Without pushing the left hip back (continue to roll it slightly forward), turn the upper torso away from the floor.
Stay in this pose anywhere from 30 seconds to a minute. Come up as you inhale, reaching through the top arm to draw the torso upright. Bring the right knee back beside the left, and repeat with the legs reversed.
Full Parighasana is a deep side bend. From the starting position described in Step 2 above, lean to the side over the straight leg. Lower the underside of the torso as close as possible to the top of the straight leg. Press the back of the lower hand on the top of the foot, then sweep the top arm over the back of the ear and join the palms. Finish as described in Step 4 above.
Benefits of Gate Pose
With any serious knee injury, kneeling might be difficult or impossible. In this case, perform the pose sitting on a chair. Arrange your legs either in front of your torso, with knees at right angles, or stretch one leg out to the side, mimicking the full pose.
As a society we are very aware and developed in our front body. We greet and explore the world with our face, front of the torso and pelvis, hands, and feet. In contrast, many of my students have told me that they experience their side body—the area from the hips up to the armpits—as a place that feels numb, dense, or heavy. Unless we get an ache in the back body, it's often forgotten about as well—out of sight, out of mind. One beauty of yoga, which means "union," is that it diminishes an emphasis on one part of the body and asks us to spread our interest and respect everywhere.
Parighasana (Gate Pose) energizes and lightens the side body and invites the breath to become truly three-dimensional. In Sanskrit parigha means "the bar used for shutting a gate." In Parighasana the body resembles that cross beam. The asana stretches the intercostal muscles that connect the ribs. When these muscles are tight, which commonly happens when we cough and sneeze repeatedly or have poor posture, the rib cage's movement is restricted, and so is respiration. Elongating the intercostal muscles improves breathing; consequently, Parighasana helps respiratory problems usually associated with asthma, allergies, colds, and flu.