BitTorrent plays a very important role in the current Internet content distribution. When BitTorrent public tracker sites are suffering from free-riding problem, private tracker sites (PTs) work very well because of Share Ratio Enforcement (SRE) which is an auxiliary effective incentive mechanism. Understanding PTs is essential to content distribution. We have crawled and traced 15 tracker sites with over 3.5 million torrents for 7 months. We first provide taxonomy of PTs, and then present measurement study on the characteristics of PTs from the user viscosity, torrents evolution, user behaviors, and content distribution. Some of the features are apparently different from public trackers. Furthermore, we analyze SRE mechanism and auxiliary credit system, and use game theory to study effectiveness of SRE mechanism. There exists "uploading starvation" phenomenon in private trackers. We model SRE mechanism and propose an improved SRE mechanism to further incent the users and enhance the performance of private trackers.