In a Nutshell, a Voltage Bucking / Voltage Boosting Transformer is a small sized option to increase / decrease system Voltage for a given piece of Equipment.
What is being done is taking a relatively small size Isolated Transformer - a Transformer with Primary windings and Secondary windings which are not physically connected together, and making an Autotransformer out of it.
The Transformers used typically have Secondary Voltages in the area of 12/24V or 16/32V, and have VA (Volt-Amp) ratings in the 250VA through 2.0 KVA ranges.
The advantage comes in the small size - in both physical size and Apparent Power (Volt-Amps), vs. the connected load.
If a certain piece of Equipment required 5.0 KVA @ 132 VAC (Volts Alternating Current), a 120v x 12v Transformer with a 500 Volt-Amp rating could be used to achieve the increase in Voltage, yet have ample capacity to pass the 5.0 KVA load - even though the Transformer's rating is only 10% of that figure - or 500 VA.
The connection scheme results in an Autotransformer being created from an Isolated Transformer.
The Primary winding is connected as normal - across line Voltage. The "Common" side is tapped to the Transformer on one side, then continues to the load, where it is terminated.
The "Other Side" of the 2 wire circuit is terminated to the "Beginning" of the Secondary winding (X1 for example) & the remaining Primary lead is tapped to the same line.
The "Output" from the Transformer comes from the "End" of the Secondary winding (X2 for example), and is the "New" lead to the load equipment.
This "New Lead" and the "Common Lead" are the 2 wire Circuit to the load equipment, which has an increased Voltage.
To lower the Voltage, switch connections so the Line Current flows through the Secondary winding - from X2 to X1, then taps to the Primary lead, then goes to the load device.
To create a Reactor (AKA "Choke") - which limits current flow via "Ballasting Action", simply connect the Transformer as Voltage Boosting, but place the terminations so they are not "In Phase Polarity".
This will boost voltage slightly, but limit current flow to a certain value - like a Ballast for Fluorescent and HID Lighting does.
Take a look at the Schematics for Voltage Boosting setups, located in the Technical Reference section.
Here are some links: Voltage Boost Transformer setups - 1 Phase Voltage Boost Setups - Open Delta Voltage Boost setups - Wye connected Buck, Boost, Choke and Polarity Tests
Here's a simple Schematic for fun!