I have taken apart two and have even made drawings of them. Unfortunately, they are in TurboCad format.
Both are made of brass and are marvels of stamping engineering.
One, probably the cheaper one, has a hole slightly smaller than the wire diameter punched in it. There are two parallel slots about 1/2" long running from the top and bottom of the hole. This creates a spring which holds the wire in place. The contact area is a bit less than the circumferance of the hole times the thickness of the brass.
The other is bent in such a way to form a right angle; imagine a piece of angle iron. The wire fits into the crotch of the angle and another bit of the brass is formed into a little spring that forces the wire into the crotch. I think this is a better contact and seems to be similar to the method used in "WAGO" connectors.
~Peter
