Bittorrent is a swarming downloading method which allows for large files to be distributed far quicker and more bandwidth efficiently than simply many users downloading from one server. When you use a torrent to download a file, you are also contributing to the files distribution. As you recieve sections of the file (which are individually verified) you will also start to send those sections of the file to other users who need them, similarly you will be receving the file from numerous other users. The more people who are attempting to download a file, the quicker it will spread.
What does all of this have to do with Knoppix? Well Knoppix is large download (700MB) and can have extremely large numbers of people looking to download it in a short space of time (when a major new version is released). Due to the load this places on the server hosting the download, Knoppix can be slow to get around onto the network of mirrors.
Knoppix 3.4 was different, in fact it was released first via Bittorrent at http://torrent.unix-ag.uni-kl.de/ . Things weren't completely smooth sailing at first as the demand was staggering but since then the setup has been refined and seems to have no problems coping with the loads thrown at it.
To top it all off, you will often see mention of people having problems with Knoppix where ultimately it is down to a broken download. If you don't have the bandwidth to not care about having to redownload a copy of Knoppix if something goes funny (and if you do can you give some to the rest of us please), then Bittorrent is the answer. If Bittorrent is happy, you can be sure the file you have is identical to the original, for modem users who may have to reconnect many times to finish the download, this is a godsend (one crash could destroy the past 30 hours downloading by http/ftp, but bittorrent will fix the download without having to duplicate any significant amount of downloading).
Now for the downside, in certain circumstances you just won't be able to use Bittorrent. If you are behind a firewall you do not control you will either not be able to use Bittorrent at all or your download speeds will be appalling. If you can control the firewall, then you should be able to open Bittorrent traffic to your computer and get maximimum download speeds.
The official Bittorrent Client is available from http://bittorrent.com/download.html though personally I prefer Bit Tornado (primarily as it has a nice pause button for when you need to do something else quickly).