El Capitan Dock
In early versions of OS X (e.g. 10.1, 10.2) you could minimize a window to the Dock and it would keep updating, e.g. you could have Quicktime movies playing in your Dock. Then at some point a few years ago they took that feature out, maybe they though it was too inefficient? But with El Cap, it seems to be back on an app by app basis.
Notice Terminal: when you minimize it, it doesn't become a freeze frame with it's icon in the corner like other windows, but it seems to enter some kind of "Dock mode" where it doesn't render the same as when it is full sized (e.g. no window controls are drawn) and it continues to update. You can see this by running Top and setting Dock magnification to maximum.
I wonder if more apps will support this in future?