The plug-in Prius kits work by faking battery status messages to the cars ECU. They make the car think that the battery SOC is high.

When the SOC (state of charge) is high, the car will easily run in EV mode below 42 miles per hour. Above 42 MPH the car will use the "extra" battery charge to assist the ICE.

So even if you do a lot of highway driving, you will still get much higher than normal fuel economy. For a few pennies of electricity.

There are a good number of these kits out there (by a few different manufacturers) and they DO WORK.

Unfortunately, with battery prices still high, they are not currently financially efficient.