When the outdoor temperature plunges below a certain threshold, usually near or below the balance point where the heat pump’s efficiency decreases, the system may activate its auxiliary heat source as a backup to maintain indoor comfort. This auxiliary heat source could be electric resistance heating coils integrated into the indoor unit. It’s important to note that modern ductless heat pumps like the Mitsubishi Electric Mini-Splits are equipped with sophisticated controls that automatically manage this transition, ensuring efficient operation while keeping energy costs in check.