All pastry jokes aside, PIE is a fully-customizable replacement for the stock on-screen navigation buttons found in more recent versions of Android. It stays hidden off-screen, only to be brought to life by a predefined gesture. PIE can be manifested on all four sides of the devices, and there's even an option to set up multiple trigger areas, making this not only a neat mod to look at, but a convenient one, too.
Of course, long-time Paranoid Android users are probably shouting "Hey, this isn't new!" – which is completely true. This has been a feature in the ROM for some time, but it's now reaching its final stages of development. Where there used to be lag, you'll only find smooth transitions and blazing speeds. The dev team re-wrote the navigation code from scratch (as opposed to simply modifying the AOSP code, like in the past) to ensure a buttery smooth experience. Judging by that video, I'd say they achieved that goal. Of course, they're not completely finished yet, so there are still a few bugs to work out.
PIE isn't just about a new navigation experience, either – it also offers customizable colors, status information (like clock, date, et cetera), notifications, and toggles. One nice thing about PIE is that it's dynamic, meaning its size will be defined by the device's dpi; in other words, you won't see a stretched phone UI on a tablet.According to PA's post on Google+ highlighting this feature, updated builds of the ROM with this freshly-baked PIE should start rolling out as early as tonight. You can head here, as well as Paranoid Android's site, for more information.
Been using PA's PIE since Alpha stage. What can I say? Completely reclaimed all screen space and getting better and better each day. The BEST 3rd party ROM I've used. Yes, better than CM and AOKP (tried both, too).