The sacrum consists of five pieces of bone when we are young; when we grow up these five bones fuse into one piece. Taoists regard the sacrum as a pump which will help to hold the sexual energy coming from the sperm/ovaries and perineum and transform the energy, at the same time giving it an upward thrust. It is similar to a way station, refining the sperm/ovaries' energy as it circulates in the body. In the Iron Shirt practice we tilt the sacrum to the back, or against a wall, exerting force to straighten the sacrum. This helps to activate the sacral pump to pump the spinal cord fluid up.
The cranium of the skull has long been regarded by Taoists as a major pump for the circulation of energy from the lower centers to the higher centers. Medical research has recently confirmed that minute movements of the joints of the eight cranial bones occur during breathing. (Fig. 2.17) Cranial movement is responsible for the production and function of the cerebrospinal fluid surrounding the brain and spinal cord, which fluid is necessary for normal brain, nerve and energy patterns in the entire body.