Partial file-sharing is important in all file sharing networks. But especially for torrents. The way partial file sharing works is:
downloaders (leechers) download different pieces of the same file, then share those pieces with each other. This dramatically speeds up downloading for everyone. It also means much much less uploading work for the uploader/seeder.

Generally, seeders/upload client will deliberately send different pieces of the file to each downloader. Thus speading the pieces of the file wide around the downloaders.

From experience, I can tell you even when a seed was not online at the time, the leechers were sharing what they had and finished or almost finished the torrent file without the seeder being needed. Not only does it help in this way, but when people are partial file sharing, download speed increases for everyone. . . At the other end of the stick, one other leecher and myself were alone without any seeds and we each had about 60-70% of the file. I was uploading to them at over 70 KB/s. They were not giving me anything in return. Yet, they had every single remaining piece I needed to finish the torrent. Freeloading kills torrents, remember that! I have seen a torrent with over 170 leechers all stuck on 99% of the file, it had become a 'dead torrent'. And I have seen torrents die within a month of creation due to 'hit and run' leechers (those who do not share the file after finishing downloading) and freeloaders. The norm is to share back to a ratio of at least 2:1 or 3:1 or better, meaning you upload (share back) more than two times the amount you downloaded. This will mean more seeders (sharers with the complete file) and faster downloads for everyone.

Some people who use file-sharing genuinely think they are downloading from websites and not other users like themselves. The ignorance of a reasonable percentage of file-sharers is rife. They do not understand the p2p concept whatsoever.

Here is a simple example of how pieces of a file are sent to different downloaders. In this example, there is one seeder and three leechers (A, B and C.) The seeder sends piece #1 to downloader "A", then sends piece #2 to "B", piece #3 to "C", then piece #4 to "A", etc. The leechers simultaneously share the different pieces of the same file with each other. This is a very simplified way to explain how it works and how important partial file sharing is to the sharing protocol and process. In reality, pieces are broken into smaller parts still. . In the end, Leechers A, B, C become Seeders A, B, C, creating a total of 4 seeders and the cycle continues with more seeders created.

All pieces sent to A are blue, B is red, C is green to show how these pieces are distributed after receiving. Each piece is numbered. In this simple example, the file is split into 9 pieces.

(Click to see larger view of the animated GiF sample image)


Some basic torrent terms: http://torrentfreak.com/BitTorrent-jargon/