In Traditional Chinese Medicine (TCM), Niu Xi 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 Niu Xi is Neutral in nature. This means that Niu Xi typically doesn't affect the balance in your body. Balance between Yin and Yang is a key health concept in TCM. Eating too many "Hot" (Yang) ingredients can lead to an imbalance whereby one has a Yang Excess. The inverse is true as well: too many "Cold" (Yin) ingredients can lead to a Yin Excess. The Neutral nature of Niu Xi means that you don't have to worry about that!
Niu Xi also tastes Bitter and Sour. 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 Niu Xi 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 Sour ingredients help with digestion and restrain abnormal discharges of Fluids from the body, such as diarrhea or heavy sweating.
The tastes of ingredients in TCM also determine what Organs and Meridians they target. As such Niu Xi is thought to target the Kidney and the Liver. According to TCM, the Kidneys do not only regulate the urinary system but also play a key role in the reproductive system and the growth and aging process of the body. The Liver on the other hand 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.