Short answer; it's the same as an eyeblink mech really.
Long answer; but the specifics of the mechanism are wholely dependant on the design, positioning on the brow and what your own personal tastes. Do you want the inner points to move up and down and have the outer ends stay in the same spot? Do you want them both to move up and down as a whole? Do you want it to be moved by a ring on some string? Do you want a lever? Do you want it moved by a finger inside the head or some external activation?
Personally, I like to experiment and work out the simplest, most efficient, ergonomic design for mechs. For example after much engineering problem solving, I made
this eyeblink. I've never really found someone else's mechanism to be what I want, but it's a good bouncing off point for my own ideas.