The section which discusses Wei Qi in some detail (along with all the other forms of qi) is "What is Qi? Qi in TCM Acupuncture Theory".
There are two main ways that you work with Wei Qi. One is to build it up - this happens from building up the overall Qi of the body in large part (which is why Wei Qi isn't mentioned specifically). So a pattern like Lung Qi Deficiency, for example, is an overall qi deficiency with a primary symptom being low immunity (i.e. weak wei qi). To rectify these issues you need to tonify the overall qi deficiency through acupuncture, herbs, moxibustion, diet, and various lifestyle changes.
The other way to work with wei qi is when you draw pathogenic factors out of the protective layer of the body. This is perhaps the more direct way, but you will see it mentioned more as "removing wind" or "releasing the exterior" and not removing from the wei qi. Same thing technically, although the wei qi is used when describing the levels of disease (i.e. Differentiation of Syndromes By the Four Levels).
The Four Levels theory shows the various levels from more shallow to deeper that diseases may follow in the body. Your treatment will change to some degree, along with the depth of needling, etc. depending on these levels. The Four Levels Are:
<ul><li>The Wei Level (Defensive)</li><li>The Qi Level </li><li>The Ying Level (Nutritive)</li><li>The Blood Level (Xue)</li></ul>
Treatment on the wei qi level, like when someone has a cold, often involves using points that "releaase the exterior" (i.e. remove the pathogens from the wei qi level) like GV 14, GB 20, LU 7, etc. and using techniques such as cupping which draws pathogens out from the wei level by opening the pores and stimulating the circulation to the surface. Conversely, you could technically drive a cold into deeper levels of the body by using too many stimulating techniques when a person is sick.