Let’s have a look at the “by the book” definition and practice of craniosacral treatment (CST), purely on the physical level.
Below a brief description of standard CST, a fundamental element of Osteopathy:

The craniosacral system develops milliseconds after conception, at the moment the first cell divides in the womb, forming the primal midline that becomes our spine, the railway of our nervous system.
From that moment onward, all our physical systems’ health is organised around the midline—the spine. The balance of every other organ and system reflects the proper structure of our nervous system and cranial flow.
Imagine a tree that has grown twisted due to a storm, causing the flow of water to weaken or stop. As a result, the water can’t reach certain branches, leading to illness or death of leaves, fruit, and flowers. Similarly, our spines, nervous system, and connective tissues are affected by tension, trauma, and injury, stemming from the prenatal and birth experiences (trauma).

All trauma affects the nervous system first! Causing the cranial gland to “slow down” its production of the Liquore cranial fluid, which in turn lowers our immune system, affecting it physically, mentally, emotionally, and energetically.

“Cranio-Sacral Therapy is an exceptionally gentle yet extremely powerful treatment, which engages with the deepest inherent healing forces within the body to enable integration of the whole body-mind. The human body has a powerful inherent healing potential. Still, this potential can be compromised or depleted by injury, disease, trauma and stress. The forces of nature, both within and outside the body, have a natural tendency to reintegrate and restore healthy function. Quantum science demonstrates that this potential arises from the electromagnetic and other forces which pervade the whole universe, which are the basis of physics, chemistry and biology, which permeate all living beings, our environment and the whole matrix within which we exist, and which are responsible for generating life, growth, health and healing. These forces are expressed in the body as rhythmic motion.
CST engages with these forces to enhance the body’s natural healing ability and to restore health and integration of body, mind and spirit.
It can enhance health in any circumstances – from newborn babies to the elderly, from birth patterns, injury, illness, severe trauma, and all the many stresses and strains of life, both physical and emotional, often resolving issues that are not helped by other means. It is a profoundly transformative process.”

Taken from: https://www.ccst.co.uk/cranio-sacral-therapy/ (highly recommended articles and explanations)