With most/all linux programs, you can learn about the options by using the manual pages or help pages. If you just type rsync, you should see that one option is partial. I just use the P which is the same thing and I think that is the one you want.
Code:
--partial keep partially transferred files
for example:
rsync -vP rsync://ftp.ussg.iu.edu/fedora/linux/core/4/i386/iso/FC4-i386-rescuecd.iso .