In cold locations the furnace must be sized for worst case coldest temperature. This means for most of the year it operates at a small fraction of it's output by cycling on and off to meet a narrow thermostat temperature range. This modification let's the temperature vary more for more efficient, longer furnace run times. Also includes an electric water heater timer so it's only on at lower cost power time of use. Both changes are not noticeable but save $500+/year for $25 in components.