you hook it up to the auto tranny computer, and it maintains line preasure in the transmission. its supposed to give you better shifts and I believe it also changes the points where the auto tranny downshifts. Ive thought about getting one, I prob will now that i have a TT. As for a NA, if you dont have a underdrive pulley yet, the money is much more well spent on it. UD pulley was the best mod I made on my NA.