It uses the principles of biofeedback (which in my mind are: that which we can measure we can learn to control) to try to get us to save gas. It tells us our current mpg, logs it and records it in novel ways to give us driving challenges.
I have found my own little obsessive game with my trip odometer which tells me average mileage, which I have stopped resetting so that I have less control over the mileage so I don't spend my whole drive worrying (little changes with large divisor make littler changes).
An even simpler game comes with the "eco mode" light. a little green indicator that tells me my car is using only 3 of it's cylinders. I recall that toyota has decided to put "eco mode" lights in all their cars (saying when you are using small amounts of gas) as they have found people drive better when they are reminded they are doing well. question of the day: compliment people when they are eating a little and they will lose weight?
Anyway, this new product provides even more encouragement for you to lose weight on the foot pedal (metaphorically speaking).
However it will lose weight in your wallet as well.
At $299 (cnet offers an alternative product that does less for $169) I would have to save 10% of my gas mileage in order to pay back the $299 in a year (assuming 10000 miles driven and 32mpg, your mileage may vary )
By plugging into a vehicle's diagnostic port, it can display miles per gallon as well as trip information and even the cause of engine problems.
But really it's designed to make your driving habits more energy-efficient.
The PLX Kiwi shows off feedback on how you're doing in that regard and gives you a "kiwi score." It's also programmed with 20 increasingly difficult challenges to optimize your score.