If you are a qigong beginner, I recommend that you do not start this training on your own. Nei dan qigong is hard to understand and experience, especially for qigong beginners. If you do not understand the training theory and practice incorrectly, you may injure yourself. Wai dan standing meditation is generally much safer. You should wait until you understand qigong and this training fairly well before you start the practice on your own. The following is for your information.
Nei Dan Sitting Meditation
Although small circulation is usually achieved through nei dan still meditation, there are several wai dan techniques that can also be used to achieve the same goal. These wai dan small circulation practices are normally done by martial artists in the Shaolin styles. For example, some of the muscle/tendon changing (yi jin jing) exercises are for small circulation. This subject is discussed in the author's book Qigong, the Secret of Youth: Da Mo's Muscle/Tendon and Marrow/Brain Washing Classics. There are many nei dan techniques for small circulation that the different qigong styles have developed. I will introduce only the one that I have practiced.
Small circulation training has two major goals. The first is to circulate the qi smoothly in the conception and governing vessels. The second is to fill up the qi in these two vessels.
I have explained earlier that there are eight vessels in the human body that behave like qi reservoirs and regulate the qi level in the twelve primary qi channels. Among these eight vessels, the conception vessel is responsible for the six yin channels, while the governing vessel controls the six yang channels. In order to regulate the qi in the twelve primary channels efficiently, the qi in the vessels must be abundant. Also, the qi in these two vessels must be able to circulate smoothly. If there is any stagnation of this qi flow, the vessels will not be able to regulate the qi in the channels effectively and the organs will not be able to function normally.
You can see that small circulation is the first step in nei dan qigong. Small circulation training will help you to build up a firm foundation for further nei dan practices, such as Grand Circulation and the Marrow/Brain Washing (Xi Sui Jing).
In order to reach a deep stage of nei dan still meditation, it is especially critical that you follow the five important training procedures: a) regulating the body; b) regulating the breathing; c) regulating the mind; d) regulating the qi; and e) regulating the spirit. You also need to know the location of the lower dan tian and the roles that the conception and governing vessels play in qigong. These are discussed in detail in the author's qigong book, The Root of Chinese Qigong. It is recommended that you study before you start practicing small circulation. Here I will only review the techniques and for more detail refer you the author's book, Qigong for Health and Martial Arts
You start small circulation training by building up qi at the lower dan tian. This is done through abdominal exercises. You must first learn how to control the abdominal muscles again so they can expand and withdraw. This exercise is called "back to childhood" (fan tong). From birth until about eight years of age, you move your abdomen in and out in coordination with your breathing. This abdominal movement was necessary for bringing nutrients and oxygen in through the umbilical cord when you were in the womb.
However, once you were born, you started taking in food through your mouth and oxygen through your nose, and the abdominal movement gradually diminished. Most adults don't have this abdominal movement when they breathe. The "back to childhood" exercise helps you to return to this type of breathing.
Once you have regained control of your abdomen, if you continue these exercises, you will feel your abdomen getting warm. This indicates that the qi is accumulating. This is called "starting the fire" (qi huo). These exercises lead the qi that has been converted from the original essence in the kidneys to the lower dan tian, where it resides. The more you practice, the easier this is to do, and the more you can relax your body and feel the qi.
Buddhist and Daoist Breathing
Breathing is considered the "strategy" in qigong. In small circulation you may use either of the so-called Buddhist or Daoist breathing strategies. Buddhist breathing is also called "normal breathing" (zheng hu xi), while Daoist breathing is called "reverse breathing" (fan hu xi). In Buddhist breathing, you expand your abdomen as you inhale and contract it as you exhale. Daoist breathing is just the reverse.
Buddhist breathing is generally more relaxed than Daoist breathing. Although Daoist breathing is tenser and harder to train, it is more efficient in expanding the guardian qi (wei qi) and in martial applications. This point can be clarified if you pay attention to the everyday movements of your abdomen. Normally, if you are relaxed or not doing heavy work, you will notice you are using Buddhist breathing. However, if you are doing heavy work and exerting a lot of force, for example pushing a car or lifting a heavy box, then you will find your abdomen tenses and expands when you push or lift, which is Daoist breathing. It is suggested that beginners start with Buddhist breathing. After you have mastered it, you should then practice Daoist breathing. There is no conflict. After you practice for a while, you will find you can switch from one to the other very easily.
Huiyin and Perineum Coordination
After you have practiced the abdominal exercises for about three to five weeks, you may feel your abdomen get warmer every time you practice. After continued practice, the abdomen will start to tremble and shake each time you start the fire. This means qi has accumulated at the lower dan tian and is about to overflow. At this time you should start to coordinate your breathing and abdominal movement with the movement of your huiyin (Co-1) (literally "meet the yin") cavity and perineum to lead the qi to the tailbone (weilu cavity).
The technique is very simple. If you are doing the Buddhist breathing, every time you inhale, gently expand your huiyin and perineum. When you exhale, you hold them up gently. If you are doing the Daoist breathing, the movement of the huiyin and perineum is reversed: when you inhale, you gently hold them up, and when you exhale, you gently push them out. This up and down practice with the perineum is called "loosen the perineum" (song gang) and "close the perineum" (bi gang). When you move your huiyin and perineum, you must be relaxed and gentle and must avoid all tension. If you tense them, the qi will stagnate there and will not be able to flow smoothly.
The trick of holding up and loosening the huiyin and perineum is extremely important in nei dan qigong. It is the first key to changing the body from yin to yang and from yang to yin. The bottom of your body is where the conception (yin) and governing (yang) vessels meet.