The princible is very different from the active M differential, but the end result is slightly similar. An open differential will always distribute the torque 50/50 between the rear wheels. If one wheel has less grip, the torque to the wheel with better grip will be reduced accordingly. By braking the spinning wheel, the differntial will see more torque, and send that torque also to the wheel with better grip.
Braking the inside rear wheel in a corner is also a way of reducing understeer, which typically happens when you accelerate out of corners (load transfer to the rear). The F20 does indeed have very little understeer at corner exits.
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