I like the mechanical analogy, here's a slightly different version. IQ is like the horsepower/torque of an engine. You might have a really fast engine but it has to be hooked up to something or it will just sit there spinning really fast making lots of noise. Rationality is learning about all the things an engine can be used to do. There are all sorts of useful modules that you didn't know existed. An engine can run anything from a car, to a textile factory, you just have to have the right modules hooked up.

Now bring it back from the analogy. Literally every single thing in human civilization is run off the same engine, the human brain. They just have different modules hooked up to them. Some modules are complex and take years to learn. Some are so complex no one is really sure how they work. Rationality training is acknowledging that exploring the space of possible modules and figuring out how to hook them up in general is probably powerful, if there is sufficient overlap between domains.