On the auto GTS, the transmission is a tiptronic, I.E. when the tranny is in "D" you pull the shift lever towards the driver's side, and the tranny goes into "M" mode. You have manual control of gear selection via buttons on the steering wheel. Much safer than shifting with the OD button, because the trans will not downshift into a gear if the motor will over-rev. It also has a Gear indicator in the lower-left portion of the speedo, so you know what gear you are in.
Overdrive is your 4th gear, it's geared in such a way that 4th gear will spin over more than it's specified gear ratio (hence the term, overdrive) it's mostly there for fuel economy when cruising. When you press the OD button to turn it off, overdrive is disabled, and the trans automatically downshifts to third (because with OD turned off, your overdrive gear, 4th, is disabled.) However, doing this can possibly overrev the motor, like, if your revving past 4.5K on the tach, it's not a good idea to do this, since when you downshift to 3rd, your gonna hit the redline. read your manual for specifics as to what gear to what RPM should be used.
There's a safer way to do this, and it doesn't involve using the OD button.
If your celica's tranny has it's OD turned on, and you are in 4th, simply mash the gas pedal to the floor, your transmission kickdown cable will do the rest, and automatically downshift the tranny into 3rd, so long as your engine revs aren't too high for it to safely do so.