Originally circuit switching worked with physical circuits. When a connection was established a complete physical circuit would be tied up by each "hop" of the connection.
However this is an inefficient way of running a network. An analog phone call needs about 4kHz of bandwidth. An uncompressed digital phone call needs about 64kbps of data rate. That isn't very much. We can easilly build circuits with a far greater capacity than that and a small number of high capacity circuits are cheaper than a larger number of low capacity circuits.
Therefore there is a desire to split each physical circuit into multiple "virtual circuitS". For this we can use frequency division multiplexing (FDM) or time division multiplexing (TDM). Generally frequency division multipexing is associated with analog systems while time division multiplexing is associated with digital systems. The circuit switching can then work with virtual circuits instead of physical circuits.
With frequency division multiplexing the signal for each virtual circuit is modulated onto a carrier (in the same way that radio systems modulate signals onto carriers). The modulated signals are then combined and sent down the circuit. At the far end the signals are seperated using frequency filters and then demodulated (again much like a radio system).
With time division multiplexing the data for each virtual circuit is sent down the physical circuit at a different time. The physical circuit will switch rapidly between virtual circuits to avoid causing undue latency. There will be headers of some sort to identify which data belongs to which virtual circuit. The virtual circuits may be either be fixed data rate with garuanteed delivery (e.g. a phone call) or variable data rate with the possibility of dropping data (e.g. a "broadband" internet connection).