Without getting too deeply into neuroanatomy, it is important to know that sensation in the SI joint is conveyed to the spinal cord (which then sends it to the brain and interprets it as pain) through what is called the lateral branch of the dorsal root of a spinal nerve.
There is also a medial branch block. The medical branch nerves serve the same function in small joints (facet joints, to be specific) of the spine as the lateral branch nerves.
During a lateral branch block, our physician will numb the lateral branch at the spinal levels that they deem appropriate by using a special x-ray called flouroscopy to visualize your specific anatomy.
In theory, if the SI joint is the source of pain, numbing the nerves that send that pain signal to the brain will cause a decrease in pain or eliminate it completely.
This is a diagnostic procedure that helps the physician to know if your pain is coming from your SI and if you are a candidate for a more invasive, longer-lasting procedure called a Radiofrequency Neurotomy or Lateral Branch Neurotomy.