BitTorrent works by dividing the file into chunks -- each with their own checksum. Clients advertise which chunks they have via the tracker, and other clients request those chunks from them. The order of preference is dependant on the ratio of how much you have shared with other clients on the network, so that clients who have already shared lots of their chunks with others get a higher priority than others.
BitTorrent is hard to shut down, because anyone can run a tracker -- there is no central server that runs all the trackers, as was the case with Kazaa. This has caused the MPAA some headaches.
There have been several 'attacks' on BitTorrent networks in the past. One was a hack by which clients 'lied' about how much they had shared with others, giving them a higher priority over others. While this slowed down the downloads of other clients, this did not incapacitate the network. As such, it was more of a hack than an attack.
Last week, there was the story of corrupt trackers which had been advertised on major BitTorrent index sites, but which sent only a load of random bytes and kept the download at 95%, thereby wasting everyone's time. Still, if you knew that a tracker was corrupt, you could steer clear of it and you'd be fine.
But yesterday came the news that HBO is 'attacking' BitTorrents of it's new show Rome. Basically, they have clients that advertise that they have the whole file, but send garbage data when asked by other clients for a chunk. Of course, with the whole checksum, it is easy for the receiving client to determine the chunk is bogus -- but due to the very nature of checksums, this can only be determined after the chunk has been downloaded. And there is no guarantee that, when you request the same chunk for another client, you do get the valid chunk then.
Eventually, due to trial and error, you will get the whole file, but it will take you a lot longer, and generate much more data traffic (which is bad news for people on a data limit).
Of course, the BitTorrent people are already on the move -- blacklisting clients have already appeared (just don't accept any chunks from someone who sent you a bad one), but if there are enough bogus clients out there, then this strategy may still make you download lots of bad chunks.
There's a lot of bitching about this, but I think it's pretty neat: HBO is using the technology against the group of people who engage in a behaviour that HBO does not want. Good for them. Cyberwarfare at its best.