Completing the pelvic girdle is the sacrum - formed by 5 separate vertebrae at the tail end of the spinal column that eventually fuses in our early 30s. The sacrum does not make any contact to any seat surface, but rather is the connecting point between the two pelvic bones via the sacro-iliac joint. It's a point of connection for numerous ligaments and some key muscles that have to do with our sitting and standing posture.
At first glance, it appears to be a rather innocuous structure - a bony bridge between each pelvic half to complete the pelvic bowl. But for something so innocuous looking, the sacrum has a great deal of duty and responsibility. The sacrum is the hub, connecting the upper and lower body. It is "city central" for energy and force, distributing the shock from the ground below as we walk/run/hop so as not to send vibrations full throttle into our delicate spinal column. From above, the sacrum also absorbs and distributes the weight of gravity of our 10 lb/5 kg head and each subsequent 24 (!) spinal vertebrae from the neck to the low back.
The sacrum takes it from all directions - and all while lending structural support to hold our guts and breathing organs in place, acting as an important attachment point for muscles for back, trunk, and lower extremities, and forming an important passage way for key nerves controlling lower limbs and basic human functions.
Given all these juggled responsibilities, it's easy to understand how the the sacrum can get a little persnickety over time. Over the course of a drummer's life, too much time on the drum throne can lead to dysfunctional posture and muscular imbalance that makes movement harder because the sacrum isn't in a happy place, both literally and figuratively.
I don't think there's a bone that is more akin to the drummer - the sacrum acting as a foundation for the human body. It's not the sexiest of bones, but it's hard to imagine any movement or action without a solid foundation. The sacrum is holding down the fort, like playing 4-on-the-floor, so that our limbs can dance about and interact with the world around us.