Alexander Hanff, who has made a series of appearances on the media to make robust criticisms of the system has posted a detailed description here. The day that BT implement this, I will leave them for another (Phorm-free) ISP.
BT Retail, who manage the ISP business have behaved quite appallingly over the matter, ignoring (and even trying so suppress) discussion on their forums - all the signs are that they are holding fire until the furore dies down. We should all try to ensure that this doesn't die down. Visit BadPhorm for more information.