Making it simple:
--cache=FILENAME -> Enable cache. Cache, for the local folder, will be stored/read from FILENAME
So, with --cache, instead of browsing through local files/folders, rsync would rely on the cache stored at FILENAME (or, if it doesn't exist, rsync would generate it)
Without --cache, rsync would ignore reading/generating a cache and just work as usual.
Mirror and backup servers are wasting a lot of IO when browsing through files/folders that will just be changed by rsync. Caching would be perfect. While browsing mailing list and google, I've found some of people looking for this feature.
I believe that I can get our company to make a donation/contribution, using paypal, to have this feature (or, if you prefer, we may contribute with server hardware which is not in use anymore). If there is any developer available, please let me know.