Private Torrents are issued by Private Trackers which require registration and often an invitation from existing members (in good standing). Most private trackers operate a forced ratio tracking system where a user is required to have a minimum amount of upload and are often required to seed their torrents for a minimum time limit to maintain their membership.
Many private trackers operate High Speed Servers (seedbox) to seed their torrents, so download speeds are often much faster. Many are also strict with enforcement of ratios and min. seeding time so it's wise to thoroughly read the rules when joining a private tracker.
Some private trackers regularly open registration to new members at random times so that new members can join without need of invitation codes. Some also have an application process where a prospective member can submit references from other private trackers to show they have a good track record and will be an asset to the community. A good working knowledge of bittorrent and IRC (internet relay chat) protocol is usually required when applying for membership, but the easiest way to join is to be invited by a current member who has earned invite privileges.
Private torrents contain ”private=1” in “info” characters. Detailed definition can be read here:http://www.bittorrent.org/beps/bep_0027.html
BitComet provides complete private torrents downloading support, it follows the rules below when downloading