MAN has introduced the HydroDrive to increase traction for off-road applications such as roads construction or log haulage.
MAN has achieved greater traction without weight being added to the truck. Because there is no transfer case, approximately 400 kg (880 lb) of weight savings can be achieved. This enables lower fuel consumption and greater payloads than conventional all wheel drive trucks. The front drive axle of the HydroDrive truck is hydrostatically driven. The driver can decide when front wheel traction is required through a simple switch in the cab. It works equally well in both forward and reverse.
It can be engaged even while the vehicle is under load. If the switch is on, the HydroDrive will automatically cut out once the vehicle exceeds speeds of approximately 30 km/hr (19 miles/hr), and will engage again if the speed drops under 22 km/hr (14 miles/hr).
The hydraulic pump is situated on the gearbox output shaft, with this feeding the wheel hub motors on the front axle. When on the road, only the rear wheels are driven (through normal mechanical transmission). In this situation, the hydraulic pump and motors are producing no friction, which again assists with lower fuel consumption. Breaking is still effective regardless of vehicle speed. The HyrdoDrive option is available on eight different MAN truck axle configurations.
Source: http://www.man-mn.com/en/Innovation_and_Competence/Applied_safety/HydroDrive/HydroDrive.jsp