The manifestation of physical symptoms without biologically identifiable cause results from disruptions in normal brain function due to psychological stress.
These disruptions become strong enough to prevent the brain from voluntarily allowing certain actions (e.g. moving a limb).
When the brain is unable to signal to the body to perform an action voluntarily, physical symptoms of a disorder arise.
However, the term psychogenic usually implies that psychological factors played a key causal role in the development of the illness.
The term psychosomatic is often used more broadly to describe illnesses with a known medical cause where psychological factors may nonetheless play a role (e.g., asthma as exacerbated by anxiety).