Relaxing HVAC usage such that the temperature increases from 75F to 78F, and then is lowered back to 75F saves energy compared with holding it at 75F. The max temperature can't be too high because the system might need hours to lower the temperature by 3 degrees. e.g. A family leaves at 7 a.m. AC turns off. The high temperature is reached around 1 p.m. Cooling resumes at 2:30 p.m. so it can hit the target temperature by 5 p.m.
I only know of two mechanisms that could make holding a temperature more efficient. Heat pumps in winter time should be left to maintain a temperature. If it is well below freezing outside the heat pump may need to fallback to a secondary mechanism of resistive heating. Therefore it's better to not setback the temperature immediately when the home is unoccupied. Variable speed fans on higher end furnaces use less energy at lower RPM, so running a system at full effect requires more power.
I only know of two mechanisms that could make holding a temperature more efficient. Heat pumps in winter time should be left to maintain a temperature. If it is well below freezing outside the heat pump may need to fallback to a secondary mechanism of resistive heating. Therefore it's better to not setback the temperature immediately when the home is unoccupied. Variable speed fans on higher end furnaces use less energy at lower RPM, so running a system at full effect requires more power.