how does re-proxy work?

[dangalist at centrum.cz]

Hello,
I was reading about reverse proxy functionality, but still cant understand how is the traffic redirected. 
For eg. if I host static content in two datacenters and one perlbal reproxy server is hosted in third datacenter.
Then is all inbound traffic routed trough perlbal server, but is outbond traffic from each datacenter routed back 
to client same way over perlbal server or the reproxy handles just direct connection between static content
server and client so the bandwidth hungry traffic goes directly from static server to client?

Goodwill