Cranio sacral rhythm
To understand the cranio sacral rhythm I need to explain about cerebro spinal fluid and the meninges.
Cerebrospinal fluid is clear colourless fluid which surrounds and bathes the brain and spinal cord.
It is a filtrate of blood and is produced in hollow spaces at the centre of the brain called ventricles.
Containing the cerebrospinal fluid is a tough waterproof sack made up of three membranous layers called the meninges which surround the brain and spinal cord.

Now, back to the cranial sacral rhythm. The production and absorption of cerebrospinal fluid makes the membrane system continuously expand and contract in a regular rhythmic motion. This rhythmic motion is known as the Cranial Rhythm or Cranial Rhythmic Impulse.
The Cranial Rhythm can be divided into two phases. The expansion phase occurs when the whole membrane system is filling with cerebrospinal fluid.
The contraction phase occurs when the whole system is absorbing the cerebrospinal fluid and contracting.
In cranial terminology these two phases are known as Flexion and Extension. The cranial rhythm has been measured as occurring normally at a rate of between 4 and 14 complete cycles of expansion and contraction per minute.