A differential stopper operates like an open differential, the wheels can rotate at different speeds. However, when traction is needed, the axles can be mechanically locked together forcing the wheels to rotate at the same speed. This is especially helpful in off-roading situations when one wheel is off the ground or on an otherwise very low traction surface. When locked, the wheel in the air doesn’t receive any torque because there is no traction and the wheel on the ground receives all the torque, allowing the vehicle to move.