The idea is to get users to seed, rather than just leach and then shut off their client. Where does saving bandwidth come into the picture?
But this isn't actually the most usrful result! In this model distributing 100MB of GoT is as valuable as distributing 100MB of some file with only 2 or 3 seeders. But those Megs are not equivalent if the objective is to maximize availability of files.
Being the seeder of last resort on a rarely (but still downloaded!) file is way more valuable than being the 100k'th seeder on some new TV show.
This isn't necessarily what the GP was getting at but is a good example of seed ratios not having the best incentives.
A couple examples:
"Required ratio" based on percentage of "snatched" (i.e. downloaded) torrents you are seeded. For example, if you're seeding all torrents you have ever downloaded, your required can be 0 (instead of initially common values of 1 or 0.9).
Ratioless: no ratio requirements, but you must instead seed all torrents for a specified time.
Among people involved in private torrent communities it's fairly well-known that a purely ratio-based system does not provide the best incitements (which would be a library of content that is as diverse and well-seeded as possible).
Anyway, the main reason many prefer private trackers to public is not only because of enforcement of contributing back, but because of rules that ensure that the available content has high quality (i.e. curation).
1. You get one point for seeding a torrent per hour, up to 10 total. You can exchange 1,000 pts for 10GB of upload credit (e.g. improves your ratio). The torrents don't need to actually be downloaded by anyone to count. This seems to work well as torrents that are weeks and weeks old still have seeds.
2. Popular torrents are listed as 'FREE', which means that downloads don't count against your ratio, but upload is still counted. So anything that gets swarmed gets lots of activity.
This in turns means that these other people don't increase their ratios and end up in the same situation. You have an artificial 0-sum game for absolutely no good reason. Then you need freeleech periods or bonuses to make up for the ratio credit "deflation".
I think it would make more sense to punish people who never seed (seeding if nobody is downloading is fine, you can't do anything about it) and incite people to seed torrents with a low seed count.
Private trackers are often topical, either focusing on movies, video games, music, ebooks etc... So if you want an invite to these you just need to hang out with people who share these interests and then ask around. IRC is a good place to start in my experience.
Some semi-private trackers also have periods where registration is open to all.
Otherwise, you have to get an invite by an existing user; if you don't know any, I think the best bet is to lurk forums and IRC channels and beg for one. I never actually went that route, though.
Most private trackers requires an invite (from someone who is already a member and has invite privileges) though.
People that brought you on were supposedly responsible for any problem you caused..
It's funny, but I happened to have a conversation with Bram Cohen about this very topic. He thought seed:leech ratios were a dumb idea, creating an artificial scarcity where none was needed. You can't save bandwidth for later.
And I write this comment from my home Sonic gigabit fiber connection, for which I'm paying exactly the same price as I paid for my ADSL connection. My bandwidth is unmetered. Other home ISPs like Comcast do meter. But is that because they need to? Or just because it's a convenient way to figure out which of their customers are willing to pay more?