It is your option to turn it off, I do in dry conditions in attempts to gain gas milage. I have not noticed any difference. To turn it off the button has to be pressed for 3 or 5 seconds. This keeps it from being turned off inadvetantly if the button is hit on accident. It will always come on at start and have to be turned off again if desired, Volvo's way of keeping us safe......................
2006 Volvo XC90 2.5
2005 GMC Sierra Crew Cab 5.3