In Traditional Chinese Medicine (TCM), Yan Hu Suo belongs to the 'Herbs that invigorate the Blood' category. Like the name indicates these herbs tend to stimulate the Blood flow. In TCM they're used to help the circulation of Blood in cardiovascular conditions or menstrual irregularities as well as to treat acute pains caused by Blood Stagnation. They can also be used to treat Blood Stagnation when it causes certain tumors, cysts and hardened clots.
Furthermore Yan Hu Suo is Warm in nature. This means that Yan Hu Suo 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 Yan Hu Suo can help restore a harmonious balance between Yin and Yang.
Yan Hu Suo also tastes Bitter and Pungent. 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. Bitter ingredients like Yan Hu Suo tends to have a cleansing action on the body by clearing Heat, drying Dampness and promoting elimination via urination or bowel movements. On the other hand Pungent ingredients tend 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.
The tastes of ingredients in TCM also determine what Organs and Meridians they target. As such Yan Hu Suo is thought to target the Spleen, the Heart, the Liver and the Lung. In TCM the Spleen assists with digestion, Blood coagulation and Fluids metabolism in the body. In addition to regulating Blood flow, the Heart is believed to be the store of the 'Mind' which basically refers to someone's vitality. The Liver is often referred as the body's "general" because it is in charge of regulating the movements of Qi and the Body Fluids. It also takes a leading role in balancing our emotions. 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.