Passive monitoring is concerned with what is happening exclusively. Active monitoring is concerned with what is happening, why is it happening, and what are the future implications. And it's doing so consciously. Many skilled pilots do this unconsciously and they call it "intuition" or even "experience". But when we break down what they are doing at the mental level what separates them out is their level of engagement with the situation. They are not merely passive receptors, middle men, a second pair of eyes. They are a second brain interacting with the world around them in ways that enhance the safety of the plane.
I used to drive a taxi. I was passive and active monitoring all the time. Physical driving using gears, clutch and brakes and at same time watching the road for other users and also watching footpaths for potential passengers. Sounds like a lot of mumbo jumbo when applied to piloting an aeroplane and normal human behaviour when driving a car.
