This is a stabilizer. which constantly monitors the output voltage and controls the variations in the input voltage by movements of a motor. This motor in turn selects the proper output voltage on the variable transformer (variac). This is probably the cheapest power-conditioning product available. It gives reasonably good voltage regulation and is all right where voltage fluctuations are not considerable. However it is not advisable to use it outside big cities where apart from considerable fluctuations in voltage, the power is full of frequency drifts, failures, noise and
spikes.