This effect shifts colours within the ZX Spectrum colour palette to create a fade effect, useful for transitions between layouts.
It can be applied to a layout to fade the entire layout, or to an object to fade out only part of the layout.
See the devlog for more information (https://little-martian.dev/21-07-05-construct-3-zx-spectrum/) and I recommend you try out the example project to see it in action.