Off the top of my head, I can think of a couple of things it tests for:
* Ability to reflect on and critique their own ideas, using technical common-sense. If someone has never had a reason to research or think too hard about how GPS works, then it might be reasonable to initially assume that it sends a signal to a satellite. But hopefully, when prompted to think about it a bit more, they would realize: "Hey, if that's true, then somehow these satellites that were launched in the 80's and 90's were able to scale up their capacity and handle orders of magnitude more demand, now that everyone has a smartphone in their pocket. Maybe that's not how it works?"
* General curiosity and interest in areas outside their field of specialty. This might not be strictly necessary to get a job done, but it probably has some correlation with other measures of technical aptitude that are hard to probe directly.
* Ability to reflect on and critique their own ideas, using technical common-sense. If someone has never had a reason to research or think too hard about how GPS works, then it might be reasonable to initially assume that it sends a signal to a satellite. But hopefully, when prompted to think about it a bit more, they would realize: "Hey, if that's true, then somehow these satellites that were launched in the 80's and 90's were able to scale up their capacity and handle orders of magnitude more demand, now that everyone has a smartphone in their pocket. Maybe that's not how it works?"
* General curiosity and interest in areas outside their field of specialty. This might not be strictly necessary to get a job done, but it probably has some correlation with other measures of technical aptitude that are hard to probe directly.