PerfectDark Unity differs from other P2P cache (such as Share) in that the Unity consists of any file being shared on the network, not just the files you are downloading. So in the background, PerfectDark will download other files into your Unity, and upload them to other peers who are trying to download those files -- even if those files aren't the ones you are trying to download (which is why your downloads will show 0.0%).
As before, PerfectDark not only utilizes shared/pooled bandwidth (which is common in multi-peer P2P such as bittorrent), but also pooled storage; and letting it build up the Unity is necessary to ensure you will be a good peer on the network.
In principle, PerfectDark requires you to have at least 2GB in Unity before it will begin trying to download your files. After your Unity reaches 2GB, it will try to begin downloading your files, but at a slightly reduced download speed. Once your Unity reaches about 10GB, you should have no limitations (aside from the number of files you can queue at a time, depending on your Unity size). A portion of your bandwidth will still be used to continue filling up your Unity, although the files you are downloading will also count towards Unity size.
But again, just because your Unity is full does not mean your files will download right away. It still depends on the file's availability, and whether or not PerfectDark needs to redirect the files through other peers before it can get them to you, and any number of other network conditions.
(´-ω-`)zZZ File is not currently active/downloading (kaomoji of a sleeping pig)
(`・ω・´)シャキーン File is currently active/downloading (kaomoji of a raging pig)
If a file in your download queue suddenly shows up highlighted in red, it means another peer has given it a bad rating, and you should take care and make sure you're downloading the right file (beware of TXT files or other small files that might have a virus, or RAR files that might have passwords which you don't know).
For uploading; I don't upload to PerfectDark so I can't help you there.