Operation Description

WARNING: This page is about a different car, the 2005 Nissan 350Z. However, it is still accessible from the selected car via links, so may be relevant.

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: Body Control Module Operation Description Diagram
Courtesy of NISSAN MOTOR CO., U.S.A.