The best description I have found so far is from a case report in the Journal of Spinal Cord Medicine who describe it thus:
Idiopathic spinal cord herniation (ISCH) is a rare cause of progressive myelopathy frequently present in Brown-Séquard syndrome. Preoperative diagnosis can be made with magnetic resonance imaging (MRI). Many surgical techniques have been applied by various authors and are usually reversible by surgical treatment.
Here is the complete case report for reference.