In Traditional Chinese Medicine (TCM), Gui Zhi belongs to the 'Warm/Acrid herbs that release the Exterior' category. Herbs that release the Exterior aim to to treat the early stages of diseases that affect the upper respiratory tract, the eyes, the ears, the nose, the throat or the skin. TCM believes that External diseases such as colds or allergies can only invade the body if the External environment overwhelms our Wei Qi (the TCM version of the immune system). In order to counteract this invasion Warm/Acrid herbs aim to induce sweating by increasing the flow of sweat to our capillary pores. The belief is that this will expel the disease from the body and stop it from invading further.
As suggested by its category Gui Zhi is Warm in nature. This means that Gui Zhi tends to help people who have too much 'Cold' in their body, although with less effect than a plant that would be Hot in nature. Balance between Yin and Yang is a key health concept in TCM. Those who have too much Cold in their body are said to either have a Yin Excess (because Yin is Cold in nature) or a Yang Deficiency (Yang is Hot in Nature). Depending on your condition Gui Zhi can help restore a harmonious balance between Yin and Yang.
Gui Zhi also tastes Pungent and Sweet. The so-called 'Five Phases' theory in Chinese Medicine states that the taste of TCM ingredients is a key determinant of their action in the body. Pungent ingredients like Gui Zhi tends to promote the circulations of Qi and Body Fluids. That's why for instance someone tends to sweat a lot when they eat spicy/pungent food. On the other hand Sweet ingredients tend to slow down acute reactions and detoxify the body. They also have a tonic effect because they replenish Qi and Blood.
The tastes of ingredients in TCM also determine what Organs and Meridians they target. As such Gui Zhi is thought to target the Heart, the Lung and the Spleen. In addition to regulating Blood flow, in TCM the Heart is believed to be the store of the 'Mind' which basically refers to someone's vitality. In addition to performing respiration, the Lungs are thought in TCM to be a key part of the production chain for Qi and the Body Fluids that nourish the body. The Spleen assists with digestion, Blood coagulation and Fluids metabolism in the body.