I think it stands for "electronic control transmission". Once in on position, it makes the auto transmission to shift to next gear at a much higher RPM (around 3500-4000 rpm in 1KZTE) making the car to accelerate faster. When "Off" it shifts quicker at lower RPM (my car never exceed 2000 rpm)to help a gentle drive and smoother shifting. This also helps maintain better fuel economy.
Electronically controlled transmissions also senses the way u press the throttle. If u floor it,the transmision will not shift to a higher gear until around 4000 revs, and if pressed gently it shifts at around 2000-2200rpm regardless of the ECT pressed or not.
Bottom line: With ECT "on" your car will remain in Power. (say u can be driven at 3000 rpm continously). Help pulling trailers or climbing hills.
I hope u have undestood (even if, 50% its okay:) )