Originally posted by bighammer
Buff, it's a switch to control the vaccuum to the front axle engagement. You can then engage/disengage the transfer case 4wd mode independent of the front axle engagement.
Probably the most common use is for a low range 2wd mode (4lo w/axle disengaged) for backing trailers or maneuvers in tight spots.
I also like it, but sort of oposite of Bob's description, for bad weather or rough going. I keep the front axle engaged so a pull of the transfer case shifter is instantly in 4wd without letting up on throttle or waiting for front axle to line up and engage.