While the preceding four vessels (Governing, Conception, Thrusting, and Girdle) are located in the trunk, the Yang Heel Vessel and the next three are located in the trunk and legs. (In addition, each of these four vessels is paired.) For millions of years, man has been walking on his legs, which preform much more strenuous work than the arms. I believe that it was because of this that, as evolution proceeded, the legs gradually developed these vessels to supply Qi support and regulate the channels. If this is true, it may be that, as time goes on and man uses his legs less and less, in a few million years these vessels will gradually disappear.
You can see from the way that the Yang Heel vessel intersects with other Qi channels that it regulates the Yang channels, such as the urinary bladder, the gall bladder, the small intestine, and the large intestine. The Yang Heel vessel is also connected with the Governing vessel. The Qi filling this vessel is supplied mainly through exercising the legs, which converts the food essence or fat stored in the legs. This Qi is then led upward to nourish the Yang channels. It is believed in Qigong that, since this vessel is also connected with your brain, certain leg exercises can be used to cure headaches. Since a headache is caused by excess Qi in the head, exercising the legs will draw this Qi downward to the leg muscles and relieve the pressure in the head.
Most of the training that relates to this vessel is Wai Dan. Wai Dan Qigong is considered Yang, and specializes in training the Yang channels, while Nei Dan Qigong is considered relatively Yin and emphasizes the Yin channels more.
The Yin Heel vessel is connected with two cavities of the kidney channel. Therefore, one of the major sources of Qi for this vessel is the conversion of the kidney essence into Qi. It is believed in Qigong society that the other major Qi source is the essence of the external kidneys (testicles). In Marrow Washing Qigong, one of the training processes is to stimulate the testicles in order to increase the hormone production and increase the conversion of the essence into Qi. At the same time, you would learn how to lead the Qi in this vessel up to the head to nourish the brain and spirit (Shen). With this nourishment, you would be able to reach Buddhahood or enlightenment. From a health and longevity point of view, the raised spirit will be able to efficiently direct the Qi of the entire body and maintain your health.
(1). Below the lateral malleolus at Shenmai (B-62) - (2). Ascends along the lateral aspect of the leg - (3). Posterior aspecl; of the hypochondrium - (3). Lateral side of the shoulder - (4). Traverses the neck - (5). Passes beside the mouth -(6). Inner canthus (joins the Yin Heel vessel and the urinary bladder channel) - (7). Ascends across the forehead - (8). Winds behind the ear to Fengchi (GB-20) - (9). Enters the brain at Fengfu (Gv-lG).
***This vessel intersects Shenmai (B-62), Pushen (B-61), Fuyang (B-59), Jingming (B-1), Juliao (GB-29), Fengchi (GB-20), Naoshu (SI-10), Jugu (LI-16), Jianyu (LI-15), Dicang (S-4), Juliao (S 3), Chengqi (S-1), and Fengfu (Gv-l6).
(1). Zhaohai (K-6) below the medial malleolus - (2). Extends upward along the medial aspect of the leg - (3). Crossing the per ineum and chest entering the supraclavicular fossa - (4). Ascends through the throat and emerges in front of Renying (S-9)- (5). Traverses the medial aspect of the cheek - (6). Inner canthus (joins the urinary bladder channel and Yang Heel vessels) - (7). Ascends over the head and into the brain.
***This vessel intersects Zhaohai (K-6), Jiaoxin (K-8), and Jingming (B-1).