PWR = Power mode, more sensitive kickdown and will hold revs till the redline.
MANU = Economy mode, gets into and holds a higher gear if possible.
Both buttons off = Normal operation of autobox.
Overdrive is basically your fourth gear, it's presence is toggled with the button on the shifter.
Best to drop it into neutral at a standstill, it takes the strain off the components.
They truly are a piece of piss to drive, just point and shoot.
Jay