Ahhh, where do I start??
A shadow is exactly that - a shady concept. I love this as it brings a whole new level of wonder and Hope to status flying.
I have followed discussions on this topic for years, and my conclusion is that I now know even less than when I started.
If I had to state what I thought to be the real case (and QF have NEVER to my knowledge officially given support to this concept), then my best guess is the following:
There is some mechanism, either auto or staff-driven, to soft-block the seat next to a status member. This sort of impedes lesser folk from selecting the seat next to you. But in true QF fashion, I suspect that at some point in time, just before the flight, this impediment is removed, and suddenly that seat is available for check-in or airport staff to fill, which they happily do.
I have had endless experiences where at the glorious heights of P1 the "empty" seat beside me became occupied by a ring-in at boarding when there were many other seats vacant for the flight. Time after time after time. The times I have indulged in conversation with the invader of my un-paid for space, they are usually someone who is on some sort of staff travel (waitlisted) or a late upgrade.
I would suggest that you should enjoy the concept, but not actually rely on it. If you choose something like a first row, then that apparently empty seat next to you has a decent chance of being filled at last minute.
Have never bought "comfort seats", but am sure they are a far more reliable way of getting an empty seta next to you.