For the impatient, the idea is this: during cooling months, set your programmable thermostat to a lower temperature at night. Your A/C will run more efficiently when it's cooler outside, and you'll "store up" the cold in the thermal mass of your house. Then, when it starts getting warmer outside, your house will stay cooler longer before it has to kick the A/C in. It helps by smoothing out electricity demand, transferring load from the higher-demand times (like midday) to the lower-demand times like night.
Of course, you have to be willing to deal with the daily variance of indoor air temperature, and the wildly varying humidity, if you're in a humid climate. I hope you don't have a piano or other string instrument!
