A common experience for many people: after cupping or cutaneous scrapping therapy (Gua Sha, 刮痧), many red and black dots appear on the treated areas, either densely or sparsely. These dots are called "sha" (痧).
What is "sha"? The human body is covered with capillaries, which are not always present in the same place. Each capillary has its own lifespan, and when it reaches the end of its life, it is broken down, absorbed, and replaced by new capillaries. However, in areas of injury or due to prolonged fixed postures, poor blood circulation often occurs, making it difficult for aged capillaries to be broken down and absorbed. They remain in place but lose their function of circulating blood, colloquially known as "occupying the toilet without doing anything."
The purpose of cupping or gua sha is to use external force to rupture these aged and fragile capillaries, allowing the body to break them down and absorb them, making room for new capillaries and thereby improving local blood circulation. The ruptured old capillaries cause punctate bleeding, which is what we see as sha spots. The more and denser the sha spots, the more aged capillaries there are in that area, indicating poorer blood circulation. If no sha appears, it means the capillaries in that area are healthy. As the number of cupping or gua sha sessions increases, the sha spots in the affected area will gradually decrease, indicating that the capillaries there are strong, elastic, and healthy.
The force used in cupping and Gua Sha is not the stronger the better. Note that the character for "sha" (痧) is composed of "疒" (illness) and "沙" (sand), meaning "small sand-like dots of illness." What we aim to rupture are the aged capillaries. If the force used in cupping or Gua Sha is excessive, it may also rupture healthy capillaries, causing harm. This is not called "sha" but "bruising." Therefore, accurately controlling the force of cupping and Gua Sha is the key to this craft. Based on the author's experience, a negative pressure of 200-400 mmHg is suitable for areas with thin skin and flesh, while 400-600 mmHg is appropriate for areas with thicker flesh. Most people can tolerate the pain, and this force is sufficient to produce sha within one minute.
Cupping is also not the longer the better. During cupping, the tissues in the cupped area become congested, and blood flow temporarily almost stops. If this condition lasts too long, it can lead to tissue hypoxia or even necrosis. Some people leave the cups on for more than ten or twenty minutes, and the affected area turns black due to hypoxia, mistakenly thinking it is sha. The author generally uses moderate force and leaves the cups on for one minute. Aged capillaries will rupture in less than one minute, and a large amount of sha will appear, allowing the cups to be removed immediately. Leaving the cups on for too long, causing the tissue to turn black, is not called "producing sha" but "hypoxic necrosis."