This mod limits you to the spawn chunk, to increase difficulty. It also builds a nice obsidian marker around your 16-by-16 area of freedom (that you cannot mine).
It works by constraining your position within the chunk, and disallowing any mining and building outside the chunk.
Mod is way too buggy
I have tested this mod with Mineclonia and can not recommend it.
This seemed true at first glance. The movement blocking felt a bit too jerky though.
When I entered a new world with this mod, I found no obsidian marker, but a bunch of server errors showed up during map generation.
The second time I entered that world, I did find an obsidian border. I assume that the placement code should wait until mapgen is done.
Mined nodes outside the spawn chunk are immediately restored, but since the nodes drop as items, this means that players have an infinite supply of everything that they can mine, which makes resource gathering absurdly easy. It is also trivial to move outside of the chunk with a boat, to explore the world and gather literally limitless resources; upon exiting the boat, the player is teleported back to the spawn chunk.