mass agent download from specific start point

[Harry Putnam]

Can anyone explain how to do this:

Mark some 30 or so groups read, up to the current posts showing by
pressing `c' 
A mnth or two later, have the agent download everything that has come
in since that time, but not before.

I have all the groups under the agent but when I press `J s' in group
buffer to start downloading... it appears the agent is trying to
retrieve every thing on the spool rather that just whats come in since
the marking read took place.

I don't really know what is happening but I see massive resource drain.

I don't want to have fuss around group by group, marking for download
or the like. 

I don't really understand how the agent keeps track of what to
download.   I guess .agentview contains that info?

I'd like to see a brief outline of how the agent carries out that task
and what marker it goes by to decide where to start downloading a
group.

I've downloaded thousands of messages over many yrs with gnus agent,
but not sure of the mechanics.