In BlockBench its possible to create cubes with negative size values. This results in a cube that only renders the further faces of itself, instead of the nearest ones, giving the illusion of a false hollow cube.

This type of cube can be used for rendering certain special effects when used in creative ways, like:

• Opaque cubes that allow rendering of things inside, similar to transparency, like a fishbowl or opaque liquids! (Unlike transparency, Clouds and water do not render over it!)
• Toonish 2D Outline effects that are only visible when the model is not overlapping its parts!
• Optical illusions for sfx

And probably much more. Polishing them out to work around the concept of "a cube that only renders the further faces of itself" can unleash a lot of homebrew tricks from the community, while still remaining as a discreet trick for those who discover you can utilize negative sizes in your favor.