A sensor has a transmitter and receiver block. The transmitter supplies the load (e.g.: a capacitor consisting of a conducting plate and human hand) with an alternating voltage signal. The sensor captures the reactive load current.
The current response is converted into a voltage and then I/Q-demodulated. After filtering and conversion, the outcome is the load impedance (capacitive and resistive information). This means that the closer the human hand gets to the conduction plate, the more capacity is measured.
This allows the detection, of not only ideal situations, but also for cases where environmental changes (e.g. a wet hand, gloves, …) have added a parallel resistive component along with a change in the capacitance.