Operation description

BCM activates transistors of output terminals (OUTPUT 1-5) periodically, and allows current to flow in turn.
If any (1 or more) switches are turned ON, circuit of output terminals (OUTPUT 1-5) and input terminals (INPUT 1-5) becomes active.
At this time, transistors of output terminals (OUTPUT 1-5) are activated to allow current to flow. When voltage of input terminals (INPUT 1-5) corresponding to that switch changes, interface in BCM detects voltage change, and BCM determines that switch is ON.

Fig 1: Circuit Diagram - Transistors Of Output Terminals
Courtesy of NISSAN MOTOR CO., U.S.A.