The major purpose of the Belt vessel is to regulate the Qi of the gall bladder. It is also responsible for the Qi's horizontal balance. If you have lost this balance, you will have lost your center and balance both mentally and physically.
From the point of view of Qigong, the Belt vessel is also responsible for the strength of the waist area. When Qi is full and circulating smoothly, back pain will be avoided. In addition, because the kidneys are located nearby, this vessel is also responsible for Qi circulation around the kidneys, maintaining the kidneys' health. Most important of all for the Belt vessel is the fact that the Lower Dan Tian is located in its area. In order to lead Original Qi from the kidneys to the Lower Dan Tian, the waist area must be healthy and relaxed. This means that the Qi flow in the waist area must be smooth. The training of the Belt vessel has been highly developed, and will be discussed in a later YMAA Book.
(1). Below the hypochondrium at the level of the 2nd lumbar vertebra -(2). Turns downward and encircles the body at the waist like a girdle.