During a fare, an uber driver told me that when he goes to an area with lots of drivers, he will turn off his app for a minute or so to artificially decrease the number of drivers in the area. If this is practiced by multiple drivers repeatedly, the result can cause a surge. He told me that he does this, and, that he holds training sessions where he teaches this practice to other drivers, among other things. I do not know if this was actually effective or if it still is.