In a part-time 4-wheel-drive vehicle, the front axles must disconnect to "freewheel" when in 2- wheel drive. When the vehicle is shifted into 4-wheel drive, the axles automatically "lock up" in the differential, providing torque at each front wheel. They are activated simply by shifting into 4WD. Manual hubs (the old style) were mechanical systems that forced the driver to get out of the vehicle and turn a switch on each wheel before they were capable of accepting engine power.