BitTorrent is today's most influential peer-to-peer system. Currently BitTorrent has two very different operating models: (1) public trackers, and (2) private trackers (also known as Darknets). A private tracker can only be accessed by its registered users, and it can provide ultra high downloading speed due to its effective Share Ratio Enforcement (SRE) incentive mechanism which stimulates the users to upload contents as much as possible. Although private trackers are becoming more and more popular, they receive little attention from the research literature, possibly because they are operated underground. To understand the popularity of Darknets, we have traced 17 private tracker sites, 2 public tracker sites and 1 BitTorrent search engine for 6 months. We investigate these private tracker sites from several aspects and try to understand why they are so successful in terms of attracting loyal users and providing high downloading speed. We then analyze the SRE mechanism, credit/point system and ratio free system used by private trackers. Our results unveil the reason of popularity and effectiveness of private trackers. Furthermore, we point out the "poor downloading motivation" phenomenon caused by the imbalance between seeders and leechers in private trackers. These understandings are essential to the sustainable development of future BitTorrent content distribution systems.