X-10 modules are used with the L2, LW, & LWM dimmers to provide wavemaking effect and control other units such as metal halides or pumps.
A X-10 control interface is connected to the dimmer using a phone cord. The interface then sends signals through the power line to the X-10 module to tell it when to turn on and off. Each control module can be set to a specific code. When that code is sent by the interface, the module will turn on or off. Both X-10 devices needs to be plugged in and the device you wish to control is plugged into the control module.
The use of an X-10 system allows for more control without having to plug numerous units into the back of the dimmer. This also allows you to use the same code to control multiple modules. For example, if you wanted to run three powerheads on A1 and four units on A2, you would need seven modules, yet these modules can be placed anywhere in your house.