They had an unsynchronized H-shifter gearbox that requires double declutching on downshifts and careful power application given the huge turbo boost. The driver has to be able to use the left and right foot for braking and be able to press all three pedals at once momentarily when downshifting under braking (left foot on the clutch, right foot toe on the brake and blipping the throttle with the outside of the foot, misleadingly called "heel'n'toe").
Here's a really good clip of Walter Röhrl putting an Audi Quattro through its paces: https://www.youtube.com/watch?v=yyVHj3sHVHQ