Are you talking about the power/comfort switch on the dash? When in power mode, it seems to make the transmission shift at higher RPMs and also seems to disable the last overdrive gear. I don't know for sure but that's what I've come to realize while playing with mine.
Just read about it and confirmed that when in power mode it causes the transmission to shift at higher RPMs, thus in the ideal powerband. Didn't see anything about overdrive though. Also, leave it in power to get better fuel mileage. Comfort is just that, it shifts at lower RPMs as to not jerk you when it shifts. Mine doesn't shift hard in power mode so I leave it there all the time.