Also known as "lamp optimization" or "tuning," the act of aligning the lamp so that its filament is at the optimal focus point in the reflector. Rarely done on a work bench--ideally the fixture is hung at a comfortable height perpendicular to a white wall or other flat surface. One possibility, in a counterweight house, is to fly in the 1st electric to a trim of about +5', and focus the fixture onto the cyc. Each brand of fixture has slightly different adjustments. While most agree that the procedure should be done every time a new lamp is installed, including on moving lights, often this is impractical.
Directions for an ETC SourceFour ERS: http://www.etcconnect.com/docs/docs_downloads/miscdocs/S4_Focus_Tuning.pdf
See also this thread: http://www.controlbooth.com/forums/lighting/257-technique-bench-focusing-360qs.html.