The principle of a variable pitch sheave is that the threaded angular faced discs forming the V-shaped groove in which the belt rides are movable. VP sheaves consist of a male with a threaded barrel, and a female with a threaded bore.
The male section with threaded barrel has flat surfaces at 180 degree increments that allow a flat surface for the setscrew to mate with. When the discs are moved toward each other, the belt rides higher in the groove and the pitch diameter of the sheave is larger which increases the driven speed.
When the discs are moved away from each other, the belt rides lower and the pitch diameter is smaller decreasing the driven speed. The common term for this speed adjustment is called “Number Of Turns Open From Closed Position”