There are a lot of really good youtube video on how the knock sensor works.
It's basically a microphone. Not all of them require a reference voltage but i believe the ones on the c/k's is. In the manual it says the 5v from the VCM is pulled down to 1.3 volts in a no knock condition, so that implies there is a shunt resistor in there to ground around the sensor.
They use a material that has piezoelectric characteristics. usually a quarts crystal in the form of a diaphragm. When the material is stretched or compressed it generates an extremely small electrostatic charge (aka a voltage) positive and negative (aka alternating). As the diaphragm moves back and forth under a vibration, the voltage produced would add and subtract to the 1.3 volts and computer would read it and decide if it is a knock or not.
The ones on the c/k's are wide banded just meaning they are not tuned to resonate at a specific vibration frequency and the computer will decide the condition. Some knock sensors are tuned meaning the crystal doesn't really get stimulated to move around except at exactly a narrow range of vibration frequency that is the knock.