This joint is where the very bottom of the spine, the sacrum, meets up on either side with the bones (the ilia) that support the hips.  It is a surprisingly common area for injury to occur, especially when missing a step or falling onto the buttocks.  It appears to be more commonly irritated in middle-aged females, possibly because of the broader female pelvis combined with the arthritic changes that become more common as we age.

It's pattern of pain production mimics "sciatica" and is commonly mislabeled as such.  Pain often occurs with standing, but can happen with sitting to.  Bending backwards, especially to the painful side is more likely to cause aggravation than bending forward.  It produces a pain, and often a burning or tingling sensation, that shoots down from the buttock, to the outside knee and sometimes further down the outside or back of the leg to the ankle.