the idea seems to be by using a setback thermostat you can automatically change the temp in the house, no need to check and set it all every time you want to reset the temp. they do indeed save you money in utility bills. your heat pump should (but some don't") already have an outdoor or ambient thermostat in the condensing unit for efficiency. it will automatically shut the condensing unit down and startthe auxillary heat at a preset outside temp.  the way you avoid the large temperature swing when you are using one of these is to turn it down an hour or so before you leave or go to bed, and up an hour before you need to have it return to the desired temp. you will have to play with the setback temps a little bit to figure out how far to set back or up. if you try to go too far with the temps you will not get back to your desired temp for a long time especially in heatpump or cooling modes.  there are stats with 4 setpoints a day, 2 up 2 down, each has an adjustable time for you to set.