Just for note, the chin spoiler moslty goes down. A splitter goes forward. You need the horrizontal plane for the excess pressure to push down (look at race preped cars like the Viper). Without it, it just opposes forward motion.
As for it creating low pressure, that requires generally low ground clearance, lower front end than rear (floor pan atleast) creating a massive diffuser, and most likely ground effects that "seal" the high pressure air from around the bottom of the car from spilling into the low pressure created under the car.
On my old Saturn I swear I felt a slight (and I may have been thinking too much) difference once the missing air damn got replaced, but several people disagree with me. That's how little difference the addition of an air dam made, and I got an extra 1 mpg, so that was probably just better driving at the time.
The Mach 1 chin spoilers appear to be just for looks, but if it is lower than the stock air damn, it may actually clean up the air flow around the motor giving less drag coefficient.
lol
