said by R4M0N:Unless you use a special hacked BT Client (which does not seed) the only time you are not going to share is when some of the peers you are connected to are Seeds and all the others in the pool either do not connect to you or already have the pieces you have. Under normal situations, you will share the pieces you have with any peer who asks you for them. The protocol is designed to have you share and the fact that you do not actually do so is dependant on what peers you are talking to and what pieces they want/need. said by RARPSL:
Since this is BitTorrent we are talking about, EVERYONE who is downloading it is also hosting those parts ("pieces") of the file that they have already downloaded. Only once you have finished downloading the full file do you go into full Hosting (AKA Seeding) Mode.
Not entirely accurate... You can download without sharing. At least you could back in the day when I tried it... I haven't used Bit Torrent in a few years now.