Fun plug is a way, how to extend the functionality of the box withou modifing it's firmware. So you don't broke your waranty. Also If something goes wrong, you can rename one file and efectively stop the funplug software from working. From that moment its a dead pice of data files, until you restore that one file back in it's previous state.

How it works? Very simply. After the device starts, it's operation system checks the presence of file named fun_plug in root of your Volume_1. If it exists, the os runs it. And thats all. This is the plug, that gives you fun to add a new functionality to your box.