A "turnbuckle" is a widely available item (link) that generally has the following properties:
1. It consists of a middle part and two end parts.
2. Each end part screws into the middle part.
3. One end part has an ordinary "right hand" screw thread, while the other end part has a "left
hand" screw thread.
Now consider an ordinary hardware-nut located in the middle of a long threaded rod. If you rotate the nut, it might be described as screwing its way toward one end of the rod, while unscrewing away from the other end of the rod.
An "initial configuration" of a turnbuckle might have both end parts only half-way screwed into the middle part. Now suppose the end parts are prevented from rotating, while the middle part alone is rotated, rather like that nut in the previous paragraph.
In this case, though, if the middle part screws its way toward one end part (part A), then because the other end of the middle part doesn't have the same kind of threading pattern as where part A is connected, end part B will also be moving closer to the middle!
And of course if we reverse the rotation of the middle part, the screwing action will cause both end parts to move away from the middle part.
I've described the ordinary operation of a turnbuckle specifically because it is relevant to this Idea. And there is one more important piece of background information that needs to be presented. See the link about "coarse thread" and "fine thread".
Over here in the USA, the non-metric bolts and nuts that are commonly available are typically offered in either of two screw-thread patterns --NOT "right hand" and "left hand", but "coarse" and "fine". (While left-hand threads are available in both coarse and fine forms, all are much less common than right-hand threads.)
While metric bolts and nuts are also available here in the USA, I've never seen any equivalent to coarse threads vs fine threads. So, any non-USA HalfBakers need to know about that before I can continue with this Idea.
Basically, a Differential Turnbuckle has a coarse thread on one end of the middle part, and a fine thread on the other end of the middle part. And both threads can be the common right-hand type.
What happens now, when the middle part is rotated and the end parts are kept from rotating, is that the middle will always approach one end part and recede from the other end part --but they will do it at different rates, because one end is rotating "coarsely" while the other end is rotating "finely".
The net effect is that the overall length of the turnbuckle changes, much like a typical turnbuckle. The difference is that the rate at which that overall length changes is MUCH less than a typical turnbuckle experiences. Which means that a Differential Turnbuckle is what you want to use when you want to make very precise length-adjustments.