When the blades have made it through the plastic sheathing to the metal wire beneath, the circuit is completed, turning on an LED indicator. The design is quite clever. Unfortunately you can't just buy one.
Brian actually made the plastic housing on a 3D printer, and has posted the plans and models to Thingiverse. So if you want one, you'll need to get your hands on something like a MakerBot first and print your own. Unless one of our clever readers can think of an easier solution, so please post your ideas in the comments for those of us lacking a 3D printer. [Thingiverse via Make]