BMW says, «the mechanical rear axle differential system optimises traction in suitably dynamic driving situations.» (ausmotive.com)
It is capable of both curbing understeer and also optimising traction out of corners (there's no limited - slip diff) and it can even learn which tyres you're using — Pirelli P Zero Corsas are standard, Trofeos an # 800 option — by calculating slip. (evo.co.uk)
The big forte of this superior technology is its uniquely precise control and engine management helping to optimise traction on rough surfaces and at the same time deliver supreme driving dynamics to the road. (press.bmwgroup.com)