I believe it disables one or more cylinders under certain conditions. Does it do anything else? Is there a noticeable loss in performance? Is the benefit in fuel economy worth it?
---- X2 ----Have just about 10000 miles on my 2019 Touring AWD. I have had the "ECO" button turned on for all except two tanks of gas.
I don't need or use "jackrabbit" starts and the car has plenty of power pulling away from traffic lights, stop signs, into highway merging, etc. I have never felt the car was under powered while using Eco mode. One of the biggest differences I have found is the transmission shifts better (no harshness, no jerks, no constant changing between or hunting gears). I have the 9 speed transmission and am very happy with it.
The other difference is while on the highway using cruise. I use the normal mode, not the ACC mode. When the car needs to speed up while using cruise, it takes a bit longer to gain speed because of how the throttle response is managed. It doesn't "floor" the pedal to get back up to speed, but instead gently speeds up. If for some reason I need instant speed, I'll just step on the gas myself.
The two times I did run without the Eco mode on, I really didn't notice much if any of a difference in fuel economy. I will say I did notice an increase in power when starting from a standing stop, but really didn't like it or need it. If I step on the gas leaving a stopped position when the car is in Eco mode, it responses with plenty of power.
Note that I am getting a life time average of almost 23 MPG, with about 80 percent local driving. Very few highway miles the last few months.