I suspect the second stroke is higher compression. First stroke has the compression "shape" controlled by the boost (miller cycle). With direct injection they

I suspect the second stroke is higher compression. First stroke has the compression "shape" controlled by the boost (miller cycle). With direct injection they could even be injecting more fuel into the cylinder for the second stroke...but I suspect the second stroke F/A ratio is determined by the first stroke remnants combined with the extra air allowed in at the bottom of the second stroke. All of this with the cam variators, timing control, boost control and fuel setup that VW already runs would be fairly easy to control with the proper sensors and code. Just speculating at this point...but it makes sense to me.