I opened one recently.
The hinge is, in fact, a C-shaped "clamp" that fits over and rides along a solid cilinder. Imagine a hand holding a Pringles can, hah.
It was very loose and wobbly. When I got to the hinges, the C was cracking at the point farthest from the gap, the arch of the C. Imagine releasing your grip on that can of delicious Original Pringles...
And that's how MacBook Airs are born...
Would you like a diagram? XD
No, but I'm headin' to the pantry in search of some damn Pringles! Thanks!