Inside the membranes of the craniosacral system is cerebrospinal fluid (CSF). This brings in nutrients for the brain and spinal cord and takes away waste matter. It also helps to cushion the brain to protect it from bangs and bumps to the head. The fluid comes in as a big fill and drains away gradually, followed by another big fill, and so on. The bones of the skull have to move in order to accommodate the fluid, so they move slightly apart creating a bigger space, and as the fluid drains away they move back together again. This movement is tiny and would not normally be felt except by a trained therapist. This tiny movement is extended throughout the craniosacral system via the membranes, and from there out into the whole of the body, probably by the nervous system.
Trained therapists use this rhythmic movement to detect abnormal tensions in the soft tissue of the body by feeling it with a very light touch of their hands, and by working in a very gentle way with this movement they help the natural healing mechanism to release the tension and return the body to ease and balance. When that happens, symptoms brought about by that particular tension start to fade away.