There are a few threads on the ECO system if you want to read up on it, although they mostly are about people asking whether there is a difference in using it regarding fuel milage.
Personally, I don't like using it as it causes too much "lag".
In a nutshell... what it does is changes your shifting points between gears earlier so that you're not "reving" too high, wasting gas as the engine is running at higher RPM's.
You'll notice that you'll shift through gears quicker, and that acceleration is slower.
Some people say they noticed no difference in fuel consumption. All I know is I don't like it... it's been off almost since day 1 - tried it for about 5-7 days... turned it off.
Does ECO have any effect on AC system? My Lexus won't allow MAX AC (full cooling/full fan) in ECO. Makes the Lexus ECO mode pretty useless in Alabama except during the winter. I hope SFS ECO isn't similar.
I quit using it several tanks ago. Am now getting 2-3 mpg better around the home area. Did not see much difference on the road but still leave it off. The turbo is much more fun to drive with it off. The better mpg is an added extra.
Forgive me, I'm not very mechanically inclined. So I could be completely wrong with this theory.
With a traditional engine, lower shift points/RPMs = better fuel economy. But with a turbocharged engine... the turbo's sweet spot with maximum efficiency is at higher RPMs, right? So... lower RPMs causing the engine to lug wouldn't be the fuel saver... higher RPMs get more turbo boost, more power, better fuel to power ratio?
So maybe that is why the ECO button doesn't have the fuel savings one might think? (At least with the 2.0T)