Borgmann:"The primary goal in development was to combine the efficiency of a six cylinder with the dynamics of a V8. We wanted the specs of a medium sized V8, combined with the fuel consumption and - most importantly - the smoothness of our six cylinder." [specs of the new six cylinder: 306 hp and 295 lb-ft]
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As a major goal were spontaneous reactions, a single turbo seemed to be too unattractive. Its reactions on accelerator movements would have been too indolent, and the characteristics too edgy at full throttle. So, the guys from Munich decided to use two turbos, each one powered by three cylinders. The advantages were much more spontaneously reactions of the smaller turbines due to smaller [and therefor lighter] rotating masses. This way, even small accelerator movements create reasonable propulsion, as little amounts of exhaust gases are enough to make the turbos spin.
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To reduce the timespan between pushing the accelerator and the reaction of the turbos even more, the engineers used another trick. Using the direct fuel injection, the turbos are being kept spinning with large valve timing overlap [I don't know the exact words in english, what I mean is when the in- and outlet valves are both open for a short period of time to create a specific effect like this one] and an air stream of unused intake air. So, even when you're cruising, the turbines are being kept spinning with some pressure.
Well sized overpressure valves are another trick to improve fuel economy. As long as you don't access power, the exhaust gases are being disposed past the turbines. The advantage: the turbos do not raise the counterpressure, the engine has an easier time pushing the exhaust gases out and uses less energy. Only when you push the accelerator, the valves close and you get the boost you need immediately.
Another part of the story when it comes to efficiency is the use of high-strength steel for the turbos. Only a few years ago, the turbos needed to be cooled with fuel, which decreased the fuel economy. Turbos made of the new steel can resist the temperatures up to 1000 °C almost without any cooling.
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Borgmann:"Even when driving progressively, this engine uses less fuel than the 3.0 NA engine [this one is based on]. Due to the higher torque, you unconsciously use higher gears all the time." That reduces fuel consumption so much that it even compensates casual power runs. Put into figures, the new six cylinder uses 2 litres less on 100 km [equals a jump from 20 to 24 mpg] than the BMW 4.0 V8. Additionally, this engine weighs 70 kg less than the V8, making the car more agile.