The link below may help, changing a bulb can be a pain.
ETC = Electronic Transmission Control
All it does is raise the shift points forcing the RPM to go higher before shifting gears.
The system is either on or off (button out or in) and no extra's or driver control such pushing the button XX times obtains another different shift pattern.
Something on the dash should light up when the switch is out, such as a PWR icon (power mode).
MPG will drop when in power mode due to shifting at higher RPM.
Why Toyota included in is unknown, perhaps give the illusion of being sporty.