the autostop will engage when the vehicle comes to a near stop, usually under 8 mph. it will remain engaged until you release the pressure on the brake pedal. this basically means that your engine is no longer running and you are using energry from the battery pack to operate your accessories. there are a few factors as to when this happens:

-vehicle has to be up to normal operating temp
-the front window defroster can't be turned on

for more info on the autostop feature check your owner's manual....