I don't know about GP specifically, but in the World championships you can cut the course as long as you don't improve your position/order. Now, how specific is that? For example, how badly do you have to "cut the course" - a knee in the dirt, a tire in the dirt, two tires in the dirt, etc, etc....I don't know. It is generally referred to when someone blows a corner and has to re-merge into the race.