I'm pretty sure it changes the acceleration rate and on my FR it seems to slow down earlier when in Eco (thus not applying the brakes) when approaching a slower vehicle.
Now I have it I wouldn't want to be without it. I do a lot of motorway miles and it's really handy. I like the way it will start to accelerate to pass a slower vehicle as soon as you put your right indicator on so it doesn't have to wait until you are completely in the next lane.
2015 Leon ST FR 184 DSG