Gua Sha means "Clear Heat/Fire" The origin of the Heat/
Fire may be internal or external but it is in the area between the skin and the membranes that cover muscles and large cavities. This is where the Wei Qi - Defensive Qi flows. The area covers the entire body and Gua sha can be done on most body parts. Its also the first area reached by external pathogens after they penetrate the skin.
Part of the normal function of the defensive Qi is to expel external Qi. If your defensive qi is deficient you may frequently be invaded by external pathogens such as wind, heat, cold, or damp. One manifestation is frequent common colds or bugs. External Qi
The external Qi has its own agenda about where to go and what to do and which isn't what your qi wants to do. This conflict produces chaotic and/or stagnant qi. Vigorous rubbing of the skin with a blunt-edged tool will open the pores and stimulate the flow of the defensive Qi. This improves the functioning of the defensive qi and it does a better job of keeping external Qi out or expelling external Qi that has entered.