Sometimes I talk about the history. How it was formed as an off-shoot from osteopathy by William Garner Sutherland when he noticed that skull sutures looked as though they facilitated movement rather than being fixed and solid. Osteopathy has changed a lot since then, although there is can be considerable overlap between the practise of craniosacral therapy (CST) and cranial osteopathy. Talking about history and differences does not seem to actually clarify the process though.
Sometimes I talk about working with fluid rhythms in the body and bone movements and how these support a system to move towards health. Fluid rhythms exist in cerebrospinal fluid, and on a cellular level every aspect of the body moves to its own tidal flow. Working with a body system to move towards its its own correct range of health is a primary function of CST.
Another explanation is around how a therapist works with a client and that each relationship is unique and drives the session. The biodynamic working model is based on a non Western model of health so explaining how it works does not fall within notions of testable repeatable patterns.
The best explanation I have found is that it is unlikely to hurt, but some sensations will arise, and to try it to find out for yourself.