Vehicles with selectable four-wheel mode can be using
free-wheel-hubs to
shut down the whole drive train (impulse-shaft,
differencial) in 2WD mode.
Advantages:
Less wear on the drive
train and the front-tires,
easier driving and less
fuel-consumption
because of the lower roll-resistance.
The manual
free-wheel-hubs have the additional advantage, that you
know,
whether they are switched on or not.
Important:
Always control both sides and turn on in time!