I think it appeared with the last system upgrade (or I hadn't remarked it before): a zone 1/5th of the size of my screen on the right hand (when in horizontal position), inviting me to drag applications there with a three finger swipe.
The text is "Drag using 3 fingers any application from one window to the other."
I don't understand what this is good for. The rest of the screen is not resizing, the underlying app is partly hidden under the side zone and therefore unusable. It would be nice to be able to have two apps in parralel. Is that how it's supposed to work?