It is a system in which force is transmitted from the engine to the wheels all the time. Such transmission is equipped with a center differential, which allows the front and rear wheels to pass freely different distances in the corners. On this car you can move in all-wheel drive mode, on-road and off-road. For heavy road conditions the center differential can be locked. In this case, all-wheel drive operation becomes similar to Part-Time 4WD, it means rigid, uniform distribution of traction between the bridges.
In some systems, the inter-axle differential lock is forced by the driver, in other ones inter-axle differential is locked automatically in case of slippage or wheel slip hazard. To lock you can use, for example, the Torsen differential type, viscous coupling, an electronically controlled multi-plate clutch and other technological solutions.