How does this anti-static mat work?

[Rodion15]

We have these mats at our workshop and I’m a bit puzzled on how they work.

These mats have two black studs on opposite corners and one silver mat on another corner.

My doubt is about the black studs: I measured continuity on these black studs with a multimeter and found that them to be non-conductive (measuring between any to spots inside the stud itself). Yet, the silver stud is conductive.
What are these black studs for then? If I connect a cord to them to an earth bonding point, then they won’t do anything!.

 

[Rodion15]

Well, I later found that there's only one valid stud, the metallic one. The black ones are doing nothing.