Think of the field pulleys as corners. You can set up a course using the drive and one, or more pulleys. Adding pulleys will not allow for longer courses. Each kit's limit is due to the size of the motor, not the length of line or number of pulleys. Adding pulleys may require a reduction in the overall course size to accommodate for the additional drag they add to the system.
Pulleys can also be strategically placed on high and low points on an uneven playing field to prevent the line from dragging on the ground or rising too high off of the ground!
For best control - position two field pulleys on either side of the main drive unit as shown in the illustration. Place the Main drive unit behind a fence so that it is not accessible to running pups. This protects it from accidental bumps and and being drug around the yard if your pup catches the lure and runs. Simply cut the line at the drive, or pick the drive up to slacken or remove the line from the pulley. Once the line goes slack, most dogs will drop the lure.