Here some short description:
There are emule user S,A,B,C on the net. Users A,B,C are using the same ISP and want the same file chunk from user S. They already entered his queue.
Now emule S decides to send the data that user A has requested and informs user A about it. User A decides to start an HTTP-proxy-download (only happens when it's needed), so he sends an HTTP request to user S through the proxy (of user A). User S sends data to A through the proxy and the data gets saved there. After this is finished, user A starts spreading the http-proxy-source (we call those packets OHCBs) to all user behind the same proxy and needing this filepart. So user B and C get the source and start downloading from the proxy. There's no more outbound traffic for the ISP anymore.
I hope this animation makes it more clear: