Until the 2008 introduction of the first all-new A4 (B8) since the A4 (B5), Audi designed drivetrains from front to back. The B5 A4 was introduced with a 50/50 power split, and because the weight distribution was dramatically forward-biased, the car plowed at the limit like it was FWD. Even the C6 A6 model in 2005 was designed like this. When Audi was designing the B8, they stretched the wheelbase six inches forward, reducing front overhang and mitigating the nose heavy feeling. To further equalize the front weight bias, Audi updated the Quattro system to operate with a 40/60 F/R torque split. This 40/60 setup had been used on a few of Audi's S and RS models as well as on the R8.
Other carmakers like Volvo still design their platforms front to back using Haldex technology, so their newer AWD systems operate in two ways. Either the cars are designed with an actively variable 100/0 F/R split or an actively variable 90/10 F/R split. Even still, only 50% of the power is able to make it to the rear wheels at any time. The systems work seamlessly, no doubt, but understeer will still occur.
To mitigate understeer further, Audi readjusted rear camber angles more negatively (like BMW's), slightly reducing the straight-line contact patch of the rear tires. The result is a still planted front end with a slightly less stable but immensely controllable rear end. This means less understeer at the limit, more oversteer (if you are skilled enough to coax it out), and incredible traction. So if you were to oversteer in an Audi, the Quattro system is designed to send power to the front wheels where there is more traction from the front tires to balance it out until the car begins to understeer. All in all, a confidence inspiring, balanced driving experience.
The sport differential is an awesome thing to have with Quattro, and completely changes the demeanor of the car. It still doesn't carve around turns like a RWD BMW, but the amount of traction available in an incredibly vast number of situations makes the Audi more controllable. This is why they always say that cars like the Gallardo and the R8 are so easy to drive. Apart from increased drag on the engine, the only downside to a car so equipped is a less rewarding driving experience. As a driver, you do less. Some like that, some don't.
2001 Audi A6 Allroad 2.7T (C5); 2005 Audi A6 4.2 Quattro (C6); 2005 BMW X5 4.4i (E53); 2008 BMW 535xi Sports Wagon (E61); 2011 BMW X3 xDrive35i (F25); 2009 Audi A4 3.2 Quattro Prestige (B8)