Sacrum Images
http://en.wikipedia.org/wiki/Sacrum

Chiropractic Perspective:

The AS Ilium Fixation, Nutation, and Respect
By Joseph D. Kurnik, DC

And on the "this is crazy" front:
SI Joint Fusion is Minimally Invasive? 
Blog post by Katy Bowman
"Just like movement of the knee keeps the quadriceps, hamstrings, and calves healthy, the movement of the sacrum keeps the glutes and pelvic floor healthy.  In fact, sacroiliac pain comes from unbalanced muscular-force between the glutes and pelvic floor, and the resulting pelvic floor hyper-tension on the sacrum.  The solution is restoring the function to minimize friction, not fusion.  SI Joint pain?  Check your butt.  Don’t see one?  Get one.  Your SI Joint will thank you."