A fundamental reason why qigong healing occurs is abdominal breathing. Just from a mechanical perspective, when you breathe using your diaphragm fully, this natural breathing method that we are borne with allows a massage of your organs in your abdominal area. And, because the pericardium is attached to the diaphragm, there is also a massage of the heart. If you can breathe deeply without forced effort and allow the diaphragm to drop and relax when you inhale, then it is also an indication that you are relaxed. Chest breathing, on the other hand, implies that there is restricted movement of the diaphragm and that you are using intercostal (between the rib) muscles to help you breathe. Those who can only do this type of breathing, breathe with more tension and are more prone to upper body muscular aches and pains.
You can check in with yourself to see if you are doing abdominal breathing by placing one hand on the upper chest and one on the belly. As you inhale, the belly should move out due to the diaphragm movement. The chest should be still. If it isn't, you will need to relax the muscles of the ribcage and allow it to drop downward. Once the chest muscles are relaxed, the diaphragm can do its natural work.
Initially, it is easiest to lie down to learn this practice. Some people even put books on the abdomen to help keep them aware of the movement of the belly during respiration. Once this breathing method is mastered lying down, you can progress to a seated posture. When seated, it is important to maintain an erect posture and to sit on the edge of a chair or on a meditation cushion. Later, if you do standing meditation, you can extend your practice in that posture. The goal is to master this type of breathing through all of your Taoist exercises, whether it be qigong, Tai Chi or Ba Gua. Integrating this natural, Taoist breathing method is part of the Way towards practicing the totally in the Tao. For a short article outlining further qigong breathing methods, read Yang Jwing-Ming's book on Embryonic Breathing.