The Paranoid Android family of custom ROMs has a history of adding interesting custom user interface elements, but since re-starting with KitKat, the developers have been trying out some new ideas. One of the first new features for the revamped PA is called Hover, and it's basically a complete rethinking of the Halo idea. Hover is a short-lived visual overlay that temporarily replaces the notification bar with a richer and more useful version, including expandable alerts and floating windows.
You know what, there's no amount of exposition that I can put here that will adequately explain this. Have a gander at this handy video from Chris Nacca.
Though Hover is currently in beta, it already looks incredibly polished, especially for those full-screen apps that require your total attention. The decision to shorten the display time to one second when tapping elsewhere is especially useful, and the floating window should work well for anyone who's trying to get more utility out of their device. For email in particular, it seems like a great way to quickly get access to expanded information.
Like most custom ROM features, Hover can't be used as a standalone app - you'll have to flash one of the latest Paranoid Android ROMs to try it out. At present PA has official builds available for all the Nexus devices going back to the Galaxy Nexus. If you're a PA user who's not a fan of Hover, don't worry, it's easy enough to ignore and/or disable. Check out the Moto X-style "Peek" function for more Paranoid goodness.
Source: Paranoid Android Google+