It's nothing too fancy and most of what it does could be done manually with greater effect, but it's a good tool for people getting started with hypermiling. First, it changes the damping on your throttle map. Where pushing the pedal down halfway in normal mode would open the throttle play 50%, in econ mode it's more like 25-30%. Second, it adjust shift points in an auto or gear ratios in a CVT to keep RPMs down (only a little bit, though). Third, it dicks around with the way your A/C works, so it's not polar cold, but still pretty cool. Fourth, it changes the way your car responds to elevation changes with cruise control on (it will float about plus or minus 3 mph from the set point in ECON mode depending on whether you're going downhill or uphill).
It's not a game changer by any stretch of the imagination and the handful of cars I've driven with it (Civic, CRZ, ILX Hybrid, TLX, CRZ, CRV) have only seen marginal gains in mpg (like 0.3-0.5, which is difficult to attribute just to ECON being on, statistically speaking). Mostly, it's just annoying and makes it hard to go through stop signs at busy intersections... I much prefer to keep the throttle map where it should be and manually control my A/C and set my own speed variance for hills depending on how high they are instead of just basing the decision on engine load.