Well, simply the sensing circuitry works through a large change in the Capacitive Charged Field on the metallic lamp it's self.
When something connects to the metallic fixture, which is a Capacitive type of connection - and has a higher level of "Storage" than the existing one [the fixture, the Dielectric Air, and some other conductor - plus possibly the earth ground too], then the circuit's overall charge gets changed.
The thing that the sensing circuit is looking for is an alternating current close to the frequency that is used to run the lamp it's self.
When it finds a high level of 60 Hz AC in the sensing circuit, it interprets this as a "Request" to change the state of the output. This connected to a 'Flip-Flop" type of gate [Binary Switches], will either turn on the load from an Off state - or the opposite.
Any body that exibits a Capacitive effect will work. I have found that touch dimmer type lamps will run the dimming control over and over when the fixture is set on a surface that is still damp from using a liquid cleaning solution on it [in this case - Windex].