In this system under normal driving conditions only one bridge is master. Four-wheel drive is connected, if it is necessary. Typically, this occurs during the slippage of the wheels and, as soon as the slip is eliminated, four-wheel drive is switched off. To connect the second bridge viscous coupling can be used or a hydraulic pump actuated multi-plate clutch, self-locking in case of difference in rotation speeds of the front and rear axle; or multi-plate clutch with electronic control, to receive information about slippage by the ABS sensors and trapping the slightest difference in the rotation of the front and rear axle speeds.
The so-called preventive system automatic 4 WD is capable using a variety of sensors (acceleration, the degree of pressing the accelerator and others) to determine the possibility of slipping and the need of connection to complete 4 WD to the drive wheels slip. It may also be provided for the mandatory inclusion of a full drive by driver.