Nadi is a Sanskrit word that means channel. Shodhana means purification. In the yogic tradition, Nadi Shodhana is thought to purify the subtle channels of the body. Alternative nostril breathing is a breathing technique that activates the parasympathetic nervous system to produce a sense of calmness. Some studies have shown that Nadi Shodhana can reduce feelings of stress as well as fostering a feeling of well being. It may also increase your ability to take fuller, and deeper breaths. As with any breath technique, if this one causes you any discomfort, discontinue. There are plenty of other techniques out there.
Try alternate nostril breathing the next time you meditate. Start by sitting in a comfortable position. Take your right hand, and fold the pointer and middle fingers into your palm. Place your ring finger near your left nostril with your thumb near your right nostril. By alternatively opening and closing your nostrils with your ring finger for the left nostril and your thumb for the right nostril you will be able to breath using just one nostril at a time.
Exhale. Close your eyes, and use your thumb to close your right nostril. Start the first round by inhaling slowly from your left nostril. Close your left nostril with your ring finger, and, at the same time, let your thumb come off of your right nostril allowing it to be open. Slowly, exhale through your right nostril. When your breath is completely empty, slowly inhale through the right nostril. Once your lungs are full, close off the right nostril and exhale through your left nostril until your breath completely empties. Inhale again fully through your left nostril. Repeat.
Try to complete ten to twenty breaths of alternate nostril breathing or more if you can. If holding your arm in front of you becomes tiresome, hold the elbow of your right arm with your left palm for support.