It's called a power divider lock ( at lest on all my IH's I have is what it's called ) . The FWD rear has a differential on the input side where the drive shaft attaches to . When you switch the PDL on, it locks that diff to ensure positive drive to both FWD and RR axle . Now bear in mind you still have a diff in each axle so only 1 wheel per axle would turn in a slick condition unless you have 4 wheel lockers (or sumsuch as I never had this lux in my old trucks
)also . Note ( as per instruction on sun visor )The PDL not to be used or engaged at highway speeds . It's a great feature to have when getting off road onto a dirt incline , snow or just operating off road .