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. Click to learn how Chinese medicine treats Premature Heart Beat.
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