The 2X Oversampler button. Is it on when it's lit or when it's blacked out?
I can see both ways as being 'on' for different reasons.
(For the record, I think it's on when it's the lighter blue in the middle - or 'lit', but it could easily be that it's set to on by default and the action of pressing it turns it off as it clears the colour in the box.)

