It goes inline with the air intake sensor, basically it fools the ECU into thinking its colder outside then it really is, which is supposed to add fuel and timing.
They say it doesn't harm the car, and it might not unless you fool the ECU too much, and make it think its 20 degrees outside when its really 100. It might advance the timing too much which is not good IMO
I'm not sure how much of a change this actual module makes, but the concept has been done long ago on many cars...
I remember people doing to '98 - 2002 Dakota R/Ts back when I had one in 2000.