The INSTEON motion sensor (available for purchase here) broadcasts wireless RF commands when they detect motion. Those RF commands are received by an AccessPoint RF and then relayed to Indigo (via the PowerLinc interface). Indigo, upon seeing this command, will update its internal ON/OFF state for that motion sensor Device. An example of this, and details on Indigo's options for motion sensors, can be seen on the Motion Sensor Basics How-To.
If you have a new and interesting way of using an INSTEON motion sensor, apply for an account (follow the instructions on the bottom of the start page) and write it up!
The INSTEON motion sensor supports three groups sending commands: