The essential thing about the Rheem Water Heater Booster is that it can get more than 45% more hot water depending on the setup.  For example, the booster can be activated when it senses the outlet temperature at 100 degrees.  At that time, the booster turns off the heating elements of the water heater and uses its own 7.2 kW element to heat the water back to 120 degrees. So basically, it works like a mini electric tankless water heater.  Since it only needs to heat the water 20 degrees, it is able to do so for about 15-20 extra minutes.
Take note that due to the algorithm written in ProTerra, in order for the booster to work properly, it can only be used in “High Demand” mode.