It seems it's just a transformer. Look at the data log on this page:
http://www.powerperfector.com/perfector.php?src=perf10It's not stabilising the voltage any, it just lowers it. They even use the word "tap".
These things are common in the States. We call them "Buck/Boost" transformers. It seems from their ad that overvoltage conditions are common. Not here. We always use the transformer in boost mode to bring undervoltage up.
Inductive loads become less efficient at lower voltages, and not linearly. I've never had cause to consider if they become inefficient at higher voltages.
I'm actually off to a meeting tomorrow morning to try and convince a developer that running his 220volt air conditioners on the 197volt supply is not a great idea. The local power company says that 197 is ok by them.