All 4WD vehicles have a transfer case between the front and rear axles that prevents the axles from spinning at different speeds. The transfer case allows the engine to provide maximum torque to the axle and wheels in order to maintain traction. Different 4WD configurations exist, and each variation employs different technologies to meet the needs of the driver. Some 4WD systems have a locking differential. This means both wheels on the axle will turn together to achieve maximum traction. Another variation is the limited-slip differential. It can detect a slide or slip in one wheel and send torque to the other wheel to regain traction. A manual hub requires users to manually engage the front wheel drive mechanism in order to fully engage the 4WD system, while an automatic hub allows users to automatically command the front wheels in order to engage the 4WD system.