Have you been seeing decreasing BitTorrent transfer speeds?
Have you received an annoying notice from your ISP accusing you of illegally downloading a Hollywood blockbuster?
Would you like to live in a better internet?
Well, I have the answer for you: encryption. You see, every BitTorrent packet your computer sends or receives contains header data stating that it’s BitTorrent traffic as well as the filename and other identifying information. By default, this data is send in plain-text, your ISP is able to intercept any traffic you send an inspect the contents (see: deep packet inspection). Your ISP may use this data to actively throttle your BitTorrent traffic (or even your connection in general if they so choose); they may also match the filename against a list of known filenames for movies or other blacklisted content and then send you (fake) legal demands.
By enabling encryption in your BitTorrent client, you make it much more difficult (individual results may vary) for your ISP to determine that a packet is a BitTorrent packet; it may also prevent you from receiving those nag letters in the mail.
Any BitTorrent client worth it’s salt will have an option buried somewhere in the preferences to enable encryption.
I’ve attached a screenshot for the client I use, Transmission:
TorrentFreak has an older with instructions for Azuerus (now Vuze), BitComet and µTorrent. The instructions may be somewhat out of date, but I’d imagine the settings would be in similar locations. When all else fails, Google it.
Update: Doug McArthur notes in the comments, enabling encryption may end up filtering out peers on less popular torrents.