What are external torrents

External torrents are torrents that are made from outside the website you are downloading from.
For example:
If your a member of Demonoid,most normal people would upload using a demonoidtracker.You will normally have the file on your computer and be sharing the file directly(this ensuring quality,allowing you to check also if its a real file.

Using an external tracker,you hotlink another tracker such as piratebay/announce a public tracker,which wont have a passkey stopping people from downloading it.It will inject seeds from the public site onto your private site(external torrents allow users to upload files with out checking them or even knowing there full contents to be checked)
Basically its a way to escape using your own bandwidth.

I personally wouldnt use public trackers/external trackers on private torrent sites due to the many rumours circulating about many main stream torrent sites striking deals with various agencies to track down internet piracy by sharing your details to a third party.