Input voltage is applied across the centre-tapped variable transformer. The potential difference between this centre-tap and the variable brush gear is the voltage applied fixed transformer.

It can be seen from the circuit, that this voltage can be in phase with the mains input or out of phase. This voltage applied to the primary will develop a voltage in the secondary winding of the fixed transformer. If this voltage is in phase with the applied mains input then the voltages are summed and the mains voltage is BOOSTED. If the voltages are out of phase then the resultant voltage is reduced or BUCKED. Hence the term BUCK/BOOST Transformer.

It can be seen that by monitoring the mains input voltage fluctuations and by adjusting the variable transformer accordingly how the output can be regulated by bucking or boosting the mains input.