Yeah I didn't quite understand the need either. Just use valve timing to bleed off pressure?Those VC engines are often (maybe always?) mated with a CVT. What could possibly go wrong?
That and I don't see the point of a VC engine with a turbo. I know it's not the same, but I would think it's more of an either or situation not both. They are only tripling down in unreliability. Not to mention that the Rogue is overpriced for what it is.
Now that I think about it, I wonder if it's to lower compression if knock is detected? I believe that sometimes very rich mixtures are used to cool things down, under WOT, which leads to more emissions and less mpg. Well, if you are going to lose power due to preignition, why not drop compression instead? Maybe this scheme allows them yet another knob to twist. Optimize pressure in the cylinder at all times.
[But when you go down a path and no one follows you... maybe it was the wrong path?]
Pure guess on my part.