It troughs and error, when no member who has completed the download is kept in the data base. Currently this means no member has completed the download after 31st of December 2014 (means 2015 or later).

In my experience the most common use is when the original is promoted after someone has posted a duplicate. And that's something only staff can do

There are two ways to push a torrent to the top of the list.
1. An automatic system "push", depending on the amount of new torrents added to the tracker. Old torrents with only a few seeders that have not been downloaded for a longer period of time will then be Recycled and added to the top of the list. This rewards long-time seeders and prevents torrents from dying.
2. A manual "push" by a moderator. Reasons may include: A new duplicate torrent but the original has very few seeders. / An old torrent that has been requested for re-seed but no help has been given. / Certain user ratio problems which could not be fixed another way / New torrents that were not initially seeded and have fallen behind. / A sort of "test" to see if a torrent really is dead.

If the information is outdated or if you have more or my translation is completely off for some reason, feel free to reply.

In short, there is not much you can do yourself to Recycle a torrent, aside from keeping up seeds for a long time while the popularity of it declines. At least that's my interpretation.