Private sites are the "scene" that you hear tossed around once in awhile. Essentially they are sites only for a select few that actually do the ripping and cracking and uploading and such, and are not connected to torrenting in any way. The only way to get in, I'd imagine, is to know someone and/or exhibit skills (cracking) or have connections (screeners, early releases, etc.) that they desire. Many of the "groups" that you see, like SPARKS, FooKaS, LEGION, etc. are scene groups, and I assume have their own sites that only they can access.
However, it is pretty much where all torrents originate from, because at some point someone takes them from those sites and puts them up on torrent sites. They also seem to take pride and gain prestige by being first with a particular rip or crack, so typically it doesn't take very long at all for it to hit a torrent site to prove that they were first. And some sites seem to have an auto-upload connection between the scene sites and the torrent site, like if you've ever looked at IPTs 0-day porn stuff. That is all clearly auto-generated by a bot, so my guess is that there is a particular scene site (KTR for example) that someone at IPT has access to and they have created a bot that automatically transfers everything over to IPT. Other sites claim to have direct scene access (sceneaccess.eu, in fact, claims that but it is obviously false because they don't have WoWS either), but in reality the torrent sites are all so closely linked and monitored that the moment something uploads one place, it is only a matter of minutes before it hits another (popular) site. Some of the smaller sites would be slower, but if it is out (on torrent sites, not scene), you can rest assured that IPT will have it a few minutes from now.
You and I will likely never see those sites. Even the auto-uploads are performed in such a way that there is no connection to the original site, so even if every torrent site in the world came down tomorrow, those scene guys would still have everything they always had.
At least, that's my understanding of how it works. I believe a_skeleton_03 said he used to be in a scene group years ago, so perhaps he can clear it up better when he's back from his vacation.