Think of the dyno as a catalyst for an actual road. If dynos didn't exist they pull the same data from the road. I couldn't tell you how the data is figured all I know is a dyno uses the same displacement as a road (or close to it). The dyno reads realtime speed and engine performance while another laptop is used to make adjustments to the tune. Them its reflashed and dyno'd again till you get the best quality tune.
At least that's how I view it.
In other news:
Car made 380whp... not too impressed. The tuner stepped out and asked where I get my gas and I said on base where its not terribly expensive. He said my car is pulling from timing because I have poor quality gas. I'm going to start filling up where I know they have better gas and flush out the BS that's in the tank. He said when I go back I can expect another 30hp from better quality gas and some more adjustments. But god damn the STI ****ing FLIES!!!!!! I did a pull in second and into third and I was blown away. Loud as **** too