This really isn't 100% medically precise, it was done just for fun! XD
Fig. 1 shows, well, the human body: it's amazing that contortionists look totally like anyone of us! Meeting them in the street, we wouldn't even suspect they're capable of curling up into a spiral or doing a 270 degrees side kick! So I guess everything's clear about this one... umm, the glowing blue thing happens to be the small intestine, it's where the food goes for digestion after saying hi to the stomach. The green thing is ascending, transverse and descending colons... and the anus! XD See [link] for a better illustration.
The spine is divided (by medics) into several regions, the flexible ones are marked (by me) with orange, less flexible with yellow.
Fig. 2 shows every region of the spine stretched to the limit backwards (and I don't mean the average commoner's limit, of course). It makes the back look creepily happy XD
The lower (lumbar) region shows what happens when the contortionist "bends in half", it's a pretty common trick among masters but as you can see it doesn't really fold the backbone in two, it's still a curve where all the 5 vertebras of the lower spine are stretched equally. This is achieved by elongating the intervertebral cartilage (the blue matter between the spine discs) through daily training. Side note: I prefer the spelling "vertebras" because "vertebrae" doesn't sound plural enough to my ear lol.
Even though it's still a curve and not a real "fold", muscles, flesh and specially buttocks help to make it look like the performer's spine is indeed "folded":
while this stretch is quite far from damaging (let alone breaking) the back and is absolutely healthy for a skillful contortionist once the blue matter gets stretchy enough.
Figure 2 also shows the maximal stretch of the neck (common in the variety of "head-sitting" postures). In some cases the thoratic region (upper back) can be more flexible but in general the mobility is limited by the ribcage.
Fig. 3 is fictional, I'd like to say that my work is precisely like sci-fi, it might only look believable but, like sci-fi, it doesn't have to be a copy of reality. Just like sci-fi writers, I try my best at being inventive while preserving the "believable" feel.
Then again, who knows, maybe in the future scientists will come up with a special medicine dramatically increasing the joints mobility. Then anybody will be able to do the splits and backbends, and contortionists would have to get their tricks on a whole new level ;P
As a little bonus, a couple of my characters showing this pose in action ^.^