Source files in use or modified while transfer

[HomeRun4711]

Hi!

I have a question on rsync's way to deal with files in use
or files that are modified on the source while they
are transferrred to the destination.

Yes, I searched the web and found

> In rsync, transfers are done to a temporary file, which is cut over 
> atomically, so the transfer either happens in its entirety or not at 
> all. Basically, rsync does "the right thing," so you won't end up with 
> partially-backed-up files. 
and

> On the destination rsync always writes to a temporary file and then
> renames it.  This operation is atomic on Unix, so processes will
> either see the old file or the new one.  Processes that have the old
> file open it will continue to use it even after it's been unlinked.
>   
Well, but this only describes how rsync is dealing with files
on the destination directory.

But what if e.g. a large mailbox file is changed on the source directory.

Is there a checksum check after the transfer and if  yes - how often is 
the file
transferred again by default.

Is rsync working with a shadow copy of the source files?

I hope you can help me.

Thanks!

Homer