Autoformers boost the voltage by transforming what comes in, reducing the amps to keep the voltage in a good range. Boosting solar controllers do the same thing and call it MPPT. The idea is fundamental electricity, watts is volts times amps. Transformers are also everywhere, those big buzzing boxes in campgrounds are transformers.
If you have a 30AMP connection that should give you up to 30amps of 108 to 130 volts of electricity, but when you put a load on it the voltage drops under 108 (considered too low for many appliances but very common in campgrounds), you need a autoformer or eventually you will lose appliances. With an autoformer the voltage will be boosted about 8 to 16% to keep your appliances safe and running right. In fact many appliances will use more watts when out of range, so the autoformer causes them to run efficiently and use less total electricity. Let us say you need to use 25amps at 108 volts, which is 25 x 108 or 2700 watts. If your connection only gives you 100 volts then the autoformer will boost it 8% to 108 volts. 100 volts boosted to 108 will cause the amps to go up to 27, still under the 30amp breaker's limit. Most autoformers have some surge suppression built in, but only to protect themselves.
We are on our second 50AMP Hughes Autoformer since a few years ago our old one burnt out. Other brands are Frank's Electronics and Surgeguard makes one, though the box is wierd and it does not have a surge suppressor, expecting you to have both a Surgeguard one and the autoformer. These units are all heavy, they need to be to handle the high loads. I know many who only buy a 30AMP autoformer since very few 50amp connections have low voltage issues, but we have seen them a couple times. We have also seen high voltage, in campgrounds with amateur electricians messing with the transformers or in places like the Yukon where the power is not as well formed as we are used to. I do not know of an autoformer that will reduced voltage, but MPPT solar controllers do it all the time.