Gathering of stuff i've read from this forum, and based on Teklom Prolog...
1 - You get disconnected daily.
2 - When you reconnect, the main authentication server checks your usage figures and assigns the appropriate IP Address. If you're over nGB, you get assigned an IP address in the "slow" range. If you're under nGB, you are assigned an IP address in the "fast" range.
3 - The routers on the SAIX backbone are configured to route "slow" IP Address ranges through a 64Kb international line, and the "fast" IP Address ranges route through an unspecified size international line.
4 - The transparent proxy processes both "slow" and "fast" IP ranges, but it sits on the international side, just before the JUMP to the UK.
5 - You are now sharing the 64Kb line with all the capped users, and probablee some other people that don't even use ADSL, like dial-up users with no international aspect to their subscription. And that can be verrrrry sloooooow.
Thats as much as I know, from what other people have said, and from tracert's output.