This arrangement can be used to either increase power, increase speed and to change direction of motion.
Increasing Speed: When the powered pulley is larger than the non-powered pulley then the speed of the non-powered pulley is higher in proportion to the circumferences of the pulleys. If the powered pulley has a circumference of 4 and the non-powered pulley has a circumference of 2 then the non-powered pulley will be turning 2x as fast as the powered pulley: This is calculated by dividing the circumference of the powered pulley (4) by the circumference of the non-powered pulley (2). When you increase speed you decrease the power (torque – or ability to do work) of the system. The non-powered pulley is moving faster but it will not generate as much power.
Increasing Power (torque): When the powered pulley is smaller than the non-powered pulley then the speed of the non-powered pulley is slower in proportion to the circumferences of the pulleys. But the power in the 2nd pulley is increased proportionally. The non-powered pulley will move slower but it will be able to generate twice as much torque.
Changing Direction: You can change direction in a pulley system by looping the belt in figure eight between the pulleys.