This is what happens from my experience (and I'm open to corrections here).
Everyone has a cap of 3GB. This means that as soon as you hit over 3 Gigs of uploads/downloads (upload = u sending information, download = u receiving information) you get assigned an ip address in the range 165.165.(1-4).xxx and you get placed on a DNS server that restricts your speed to international servers.
The restriction however is so strict that trying to connect to an international server is virtually impossible or so slow that you get fed up waiting for a page to load (which may take hours) that you cancel the request.
Now from what I gather and from what I've read in previous postings, you can get away with an extra 1 Gb (roughly) of bandwidth if you are close to your cap and don't disconnect for the duration of IP assignation. In other words, say I'm on 2.9 Gig of total bandwidth on tuesday night. If I connect up on Wednesday morning and download the whole day without disconnecting, I'll get away with whatever bandwidth I've used for the day or for the duration I've been connected. The next time I connect up, I'll be capped.
To elaborate further, the cap is only enforced for any international access. This means that even though I'm capped, I can still access local (South African) servers and pages. This also means that I can still download as much as I want (at a pretty decent speed) and send e-mail, play games or do whatever I want to do, but only on local servers. I don't know if this holds for any server in Africa but my gut feel is not.
Now to describe how the cap works: any bandwidth is added to the cap. So local and international bandwidth both clock up. This means that if I access only local sites, I'm eating away my access to international sites. This is a huge moot point and my opinion is that local access should not be clocked up as bandwidth - cap us on international only (upload and download). To have local bandwidth added to our usage is unfair.
I hope this answers your question. If I've left anything out, you guys are welcome to add on to the explanation.