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Mkoesel,
I do not know the system, but the main goal is to continue venting exhaust gases right? And to do it while decelerating, you need to eliminate or really limit compression while keeping sufficient exhaust volume flowing. My guess is that the valvetronic system is being used to independently
Simultaneously, you need to keep the turbo unloaded so that it keeps spinning but is not loaded down with compressing air.
So my guess is, based upon the descriptions I've seen thus far:
BMW is continuing to inject fuel and air into 3 cylinders during decel, keeping the intake and exhaust valves open to optimize exhaust flow.
My guess is they are then keeping the wastegate open to vent compressed air to enable the turbo to spin at maximum speed.
...
As for why this has never been done before: I think that's like asking why Nissan was one of the first to offer rev-matched downshifts on their manual transmissions in the 370z.
I think it became possible in the last 10 years and no one has incurred the design cost to do it. Because to your point, all the components are there.
However, it may also require unique turbocharger design...and many manufacturers use a nearly pre-existing turbo.
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