So what's play as you download? Well on the Playstation 4, you can buy games through the Playstation Network, and that allows you to download games directly. This varies from small, little Playstation Network titles, to full, complete games like Killzone or Knack. And play as you download allows you to actually jump into those games before the games are completely downloaded.
The way it works is that the game will download the initial levels, whether it's a tutorial or the first world, or however the game's structured. It will download that first, allow you to jump in and start playing, while the rest of the game downloads in the background. And what's great is that it's very much behind the scenes. You actually don't have to see much of it, and whether or not your connection's fast enough and you're downloading fast enough, there might actually not be much of a wall at all between when you finish that first level and you jump into the rest of the game.
So play as you download is actually a really innovative and useful new feature built into the Playstation 4 that really minimizes the time it takes between you getting a game, and you playing a game.