syncro is a fully mechanical system that uses a visouc coupling. Itsfull of plates and when they spin at different speeds the fluid heats up and locks the plates In a fourmotion the system is called haldex. It uses sensors to detect slip and alter bias accordingly. With Haldex you can get it mapped for more rear bias or user adjustable to requst the percentage as required. I think there is a limit percentage wise of about 60/ 70% but dont quote me on that... The syncro system can in theory put 100% to the rear (if wheels are of the ground but obviously in real life its nowhere near that.