> How does httrack check for updates in a site?  Does it
> d/l the entire file, or just check the size or
> something? 

It does rescan the whole structure, sending update requests 
to the remote server. Upon detection of modifications, new 
files are transfered to replace previous ones, and the 
structure is rebuilt according to new remote files. Update 
requests are faster than regular requests, and therefore 
this process is generally much faster than the first 
download.

But note that some servers are sometimes unable to respond 
properly to update requests, and ALWAYS send "fresh" data, 
spending more bandwidth than necessary. It is, 
unfortunately, impossible to avoid that ; update handling 
is done remotely, and the client can not control everything.