Pretty sure you are referring to the SelectShift transmission -- as above, if you put the truck in "M" (manual), then you use it to shift gears up/down. If you are not in "M", then you can also use it to see what gear you are in (on the LCD screen), and you can also cancel higher gears (e.g. cancel 6 and 5, then the truck will only use gears 1-4).