Cf Debian bug: http://bugs.debian.org/539816
Manual page reads:
This forces the block size used in rsync’s
delta-transfer algorithm to a fixed value. It is
normally selected based on the size of each file being
updated. See the technical report for details.
However, according to comments in the Net, this can drastically
improve rsync performance on fast networks (1Gb etc). Please include
suggestions and more explanation how would bigger block sizes have
impact on improving the performance of backups.