Did you read that article in the link? It tells what air gap is - the distance between the flywheel and the clutch disc. You really can't measure it without pulling the bellhousing off. The way the stop works is that it keeps the pedel farther up, which keeps the clutch disc closer to the flywheel, so that when you dump the clutch, the clutch disc has less distance to travel, thus it contacts the flywheel sooner, transfers power from the motor to the drivetrain faster, which gets powers to the tires faster, which creates a faster reaction time and a faster ET.
Seems to me like you could get an adjustable clutch cable and just engage the clutch at a lower point to get the same effect, but I guess this is for stock-style clutch cables.