Isn't that mostly going to make you go through more fuel during city driving?

I have read where people said mileage actually increased because you don't have to hold the throttle down to get the desired effect.

How does that work? All the thing does is basically press the pedal down harder for you.

OK, the throttle and pedal work off two position sensors in each unit. The voltages are not the same between the two sensors in each pedal. While one sensor may read 4.5V in an ascending scale when the pedal is being pushed, the other may be reading 4.4V in a descending scale. Different companies do the voltages differently but the important thing is they don't match but must be seen by the PCM in whatever format it's supposed to be or the PCM shuts the throttle body down so you don't get a run away car.

So the only thing this pedal commander can really do is address the voltages the PCM sees from the pedal sensors to alter the opening of the throttle blade angle in relation to the pedal. The throttle blade actuator is duty cycled. In other words there isn't a constant voltage being applied to it, instead it's rapidly applied on and off in a percentage of time. Like slow pedal application may be applying voltage at a 40% rate while rapidly applying the pedal may be at an 80% rate of duty cycle.

So this is what the pedal commander is altering. By changing the voltage rate application of the pedal it's telling the PCM to apply more duty cycle to the throttle blade actuator, opening it quicker. I hope I explained that well enough.