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.