Folding magic is a general category to describe one of the most interesting aspects of magic for me: teleportation, gates, and pocket dimensions. However, I intentionally made it a rare form of magic, along with healing, because I feel that both of them are used to shortcut stories, reduce tension, and basically bypass plot with easy answers. Call it the giant eagle plot of Lord of the Rings or a desire not to map the entire world (I really should have made speedsters also difficult).
The inspiration of folding magic came from two places: my mother's D&D game and Simon R. Green's Hawk and Fisher series. Simon had a plot where drugs were made in a pocket dimension that collapsed as they got rid of the source. My mother had the pinnacle of magic being the ability to create a redoubt, a pocket dimension that acted as a on-demand storage and a place to safety.
It took me four years of steady gaming (and somewhere around level eighty, let's me honest, she was barely playing D&D at this point) before I got my first redoubt and I was so happy. It was months of questing, making artifacts, and building until I got it. But that joy was something I hope to always remember.
I want it to be that rare. Yeah, one of my favorite characters (so far) will eventually create a redoubt but it will be out of the range of most people skills. More so after the Mechanical War.