Complex sentence: This release allows free exchange between CSF and nervous tissue of brain and spinal cord, which is why sampling of CSF (e.g. through a "spinal tap") provides a window to the CNS.
Split into simpler sentences: This release allows free exchange between CSF and nervous tissue of brain and spinal cord. This is why sampling of CSF (e.g. through a "spinal tap") provides a window to the CNS.