The AFM is automatic. It will kick in and basically deactivate cyclinders when the power isn't needed. There isn't anything you can do manually (short of modifying the car) that will allow you to turn it on and off when you want to. A good example is this: you are driving down a highway and running at 65mph. You don't need additional power so cyclinders will be deactivated. Now you need to pass someone and you step on the gas to go up to 75mph. The cyclinders are now all required and are all active. When you reduce the need for power, the AFM kicks back in. Your fuel economy is a function of a period of time and your driving characteristics.
http://en.wikipedia.org/wiki/Active_Fuel_Management