I wondered what ECT was when I purchased my 1995 4Runner used so I looked it up in the manual. All it said was that it changes the shift points a bit to give more power when needed. As far as I can tell, what it does is simply shift at higher RPM's. In theory, that should give you worse gas mileage. So, off equals conservative shifting, on equals higher RPM more aggressive shifting.
Here is what Yotatech says about ECT: "The ECT button stands for Electronically Controlled Transmission and it essentially changes the shift points. It will let the engine rev higher before shifting to the next highest gear. This is useful for faster take offs and climbing those mountain passes. Again, use of the ECT button can diminish mileage. "