The system seems to react differently for different inputs, also depending on the drive mode. We have a lot of snow and icy conditions here in Finland so it is quite easy to find out some of the logic what the system does in different situations and drive modes. The clutch engaging the front wheels is operated electrically, and there is also a computer trying to predict some situations.
This is why these modern systems are called intelligent: The main AWD engaging factor is not the traditionally used wheel spin, but different for every driving situation and different driver input (gas pedal input, steering angle, steering input, possibly even outside temperature). The system tries to predict when FWD might be needed and disengages it when fuel economy is most likely desired. Wheel spin is still there as a backup factor, but usually the front wheels are engaged before any wheelspin happens.
F.ex. when starting from standstill, FWD is engaged in all drive modes except Eco. And when driving on highway, FWD is dropped out for fuel efficiency, except for slippery drive mode. When cruising on RWD and stepping on the gas pedal, the front wheels are engaged quickly, before downshift and engine power output. The result in most situations is that we get the best part of RWD (fuel efficiency on highway) and very similar on-road performance to true 4WD (FWD engaged before power output).
Off-road/snow drifting/racing are a different story.
