This is a cool question. I think if you look at cross section of a turtle’s skeletal structure, it becomes easier to understand what happens inside the shell when the turtle retracts its head. It would appear to be less of a retraction, and more of a reversed nodding motion.

The spine is a part of the shell of a turtle, therefore it does not move when the turtle retracts its head into the shell. I think the majority of what moves when a turtle retracts its head is the extra skin and muscle around the neck.

Also, this exact question was asked on answerbag, apparently.

Some one buy bezdomnaya a cigar, she is correct!!

Sure! Look out, cigars away! And for some reason she is a homeless woman in Russian.

If you look at sideneck turtles (Pleurodira) it is very obvious that they flex their necks in a lateral S-shape. Other turtles do a vertical type of flexion.

As Darwin mentioned, it depends on the type of turtle.

@syz I totally did not realize that @Darwin had answered right above you, so I thought you were talking about Charles Darwin like he was your homeboy. Almost fell out of my seat laughing!

@oratio Homeless indeed. Most people do not get the moniker, understandably.