The UK power plug (and its matching socket) is incredibly safe, just as you’d expect from a country that only allows half-voltage sockets in bathrooms (bathrooms also get string-activated light switches lest your wet hands come near dangerous electricity). The problem is that it’s also big and bulky thanks to the mandatory inclusion of a fuse and an earth (ground) prong in every plug, even those meant for low-power use.
The Thin Plug aims to fix that.
A UK plug has three prongs, two carrying power and one earth, or ground. This third prong is longer than the other two and will trip a mechanism inside the wall socket which opens the holes for the live and neutral prongs. Without an earth prong, you literally can’t plug it in.
Inside the plug is a cylindrical fuse, most often rated at 3A or 13A (although 5A versions also exist). The fuse is required to complete the circuit on the live part of the plug, and so without a fuse, nothing will work.
Finally, all power sockets have individual on-off switches. If you have a strip with two sockets, it’ll have two switches. Extension cords don’t need these switches.
The Thin Plug (seen here in two fine photos from Fraser Speirs) puts the main two prongs in-line, just like a regular two-prong plug from Europe, but achieves a thin body thanks to its collapsing earth prong. In the open position, it locks and looks something like a non-threatening scorpion tail. When closed, it sits between the other prongs, although it does stick out quite a bit beyond their tips. Fraser says that the prong doesn’t lock when closed, either, so it might be susceptible to damage if it opens up in a bag (he’s only been using it for a few days so far).
It’s an ingenious workaround for an awkward problem. If you ever saw a UK “duck head” fitted onto a standard Apple charger then you’ll know just how absurdly big the thing is. But much as I hate them, I’d rather have an overly-safe UK plug on my cable than one of those terribly flimsy things you use over in the U.S. Seriously — you’re supposed to be the most powerful country in the world (after China) and yet you have these crappy plugs that feel like they’re made from aluminum foil.