The desk sends white noise to each and then both sides of the PA with a special high quality microphone positioned in the 'audience'. It works out the frequency response of the pa + room and any resonances automatically eqs them out. That means you can use the same settings in different venues, even using their pa, and get pretty much the same sound. Obviously tweaks may be needed and in venues where guitars and bass are backline only we need to do our own adjustments (for example on Saturday I was near a corner and had to cut my bass control, which I normally have slightly boosted).