Zero documentation on this that I've seen but I use the purge plate to do a initial extrude (to push out any burnt filament/get the stream flowing nicely) and then wipe it off on the edge of the build platform. The purge plate bellow catches my extruded filament and allows me to do my printing preparations in a simple, automated way ^_^
You can see the gcode for using the purge plate in my skeinforge start.gcode file:
github: start.gcode