Engine drag torque control prevents your wheels locking on slippery surfaces if you take your foot off the accelerator suddenly or shift down a gear fast. The braking effect of the engine can cause the driven wheels to skid when they temporarily lose traction. In such situations, engine drag torque control maintains directional stability and boosts safety. The system's control unit receives information from the wheel-speed sensors and the engine control unit or transmission control unit via the CAN data bus. If it detects wheel slip, it sends a signal to the engine control unit to increase engine torque, until the driven wheels are turning at a rate appropriate to the car's speed. This keeps your car steerable and maintains directional stability.

Article source: www.volkswagen.co.uk