Metaworlds looks to be one of those mods. Before I explain any farther, just look at this simple picture:
the mod's forum page, it's basically a game save file within your game save file - a sub-save... or "metaworld" in this case.
These metaworlds can be edited directly in-game, allowing you to move them around, make them rotate or fly, and even scale their size! If that's not crazy enough for you, try this - everything still functions. Chests, furnaces, and yes even redstone all work regardless of position or size!
Here's Captain Sparklez showcase video where he puts the mod in action. It's pretty wild stuff.