Below the translucent window on the top of the unit there is a sensor that registers ambient light level.
Top left, there is a knurled wheel that adjusts the light level at which you want the lights to come on.
At it's most sensitive setting it will even switch on the lights when passing under a bridge in broad daylight, then off again when you come out the other side.
The switch on the r/h side is to select either side or headlamps, and the button on the top is the on/off selector.
Once you have set it to your preference, if you have set it to headlamp mode, and leave switched on, you will never need to turn your lights on or off again.