It is just a step down transformer you wire in series with the load.
If you wire one end of the secondary to the line and the other to the load it will either add to (boost) the voltage impressed on the load or subtract from (buck) the voltage, depending on the polarity of the connection.
Say I have 208 and I need 240 I have a transformer giving me 32 volts to add to the 208. If I am going the other way (240>208) I use the same transformer wired the other way to "buck" it down to 208.
[This message has been edited by gfretwell (edited 03-15-2006).]
