Some materials require static boolean inputs to apply effects. They’re easy to setup in base materials, but don’t translate into instances because they’re static. To give users the option to enable such booleans in material instances, we can use Switch Parameters. Here’s how to set them up on the base material:
Instead of supplying the static boolean directly on the input, we use two of them (one true, one false) and plug them into a Switch Parameter. This in turn goes into the boolean material input. We can supply a default value (true or false).
When viewed in the Material Instance, we see two tick boxes. One to enable the override, and the other to change the value (either to enabled or disabled).
Quite clever and easy when you know this, but seriously difficult to figure out as it’s not well documented or intuitive.