yeah, it should be pretty obvious if you take a look at them. but you are asking so it's like this:
The spring has a fixed length under the static load of the car, the top perch (the ring that the top of the spring sits against) is fixed too. So the only thing that can move is the bottom perch (the ring on the threaded section)
So to jack the car up, move the bottom perch up the threads, to lower the car move it down. They work good because you only change the position of the spring perch, the spring length and shock position stay the same so you don't lose travel or change spring rate when you lower the car (within reason).
Now, there might be more to it depending on the brand, because when you're driving and hitting bumps there's vibration and everything else and it'll move that bottom perch, which would mean you'd go set your ride hieght and be off driving then the car would slowly lower itself over time until you lay frame, that's bad btw
So they'll be a locking system of some sort, either two nuts making up the bottom perch. The top nut will be the perch, the bottom nut gets tightened into it to lock it in place. Or the bottom perch will have a split in it and they'll be a small screw holding that split together, tighten the screw to clamp the perch onto the threads. That's the ways I've seen it, their might be something different out there too.
If you can't figure it out and have no luck then pull the wheel of and take a picture and we can all have a look-see.
