I have tried Dave’s method on Mac and can’t get it to work either. I also notice that when Dave moves the copied square, it appears to move in a perfect horizontal line. My experience is like @paul.mcalenan where the copied item moves around randomly until you find a locking point. But that doesn’t happen if you haven’t moved the handle. The copy will then move along an axis automatically.
I think we need a photo of Dave’s tongue position when making this clever manoeuvre.