Short answer: Nikon isn't saying.
However, we do know this: the Z6 II, Z7 II, and Z9 with current firmware are the only cameras that "have" it, and with most VR lenses they achieve about a half stop extra benefit when measured with CIPA standards.
The likely answer is this: the dual-EXPEED6 and EXPEED7 cameras have enough extra CPU horsepower to allow Nikon to create slightly more sophisticated algorithms for combining the lens VR and sensor VR functions.
