Above is an animated model demonstrating the theoretical coupling motion between the cranial bones and sacrum.  Movement of the cranial bones is thought to occur between the sutures and is reported to be less than 1/100th of a centimeter (the width of a human hair).  A cranial lesion can lock the sacrum in either a nutated (anterior) or counternutated (posterior) position.

The concept of a fused skull is erroneous, dating back to a paper written in 1873, known as "The Monro-Kellie Doctrine".  (This doctrine states that the central nervous system and its accompanying fluids are enclosed in a rigid container whose total volume tends to remain constant. An increase in volume of one component, e.g., brain, blood, or cerebrospinal fluid, will elevate pressure and decrease the volume of one of the other elements.) 

The research supporting this doctrine is scientifically flawed and would not pass peer review scrutiny to be published today.

             

Cranial Flexion   -    cranium is shorter and wider
Cranial Extension - cranium is longer and narrower

Glossary Of Osteopathic Terminology

Usage Guide, April 2004, Figs 57-8, pp 24

Bert and Ernie Caricature - Sphenoid bone locked in Flexion (Ernie/orange face) or Extension (Bert/yellow face)