I'm not sure. It does nothing much. It's about 7GB in total. 
here is what log says:
2013-02-15 20:00:00 incr backup started back to 2013-02-11 20:00:00 (backup #7) 
for directory /
2013-02-15 21:21:18 incr backup 11 complete, 10852 files, 621446397 bytes, 0 
xferErrs (0 bad files, 0 bad shares, 0 other)
2013-02-15 21:21:18 removing incr backup 4

While this is from the last week when it first started:
2013-02-04 20:00:02 full backup started for directory /
2013-02-04 20:41:21 full backup 0 complete, 332200 files, 7697251897 bytes, 0 
xferErrs (0 bad files, 0 bad shares, 0 other)
2013-02-05 20:00:00 incr backup started back to 2013-02-04 20:00:02 (backup #0) 
for directory /
2013-02-05 20:11:43 incr backup 1 complete, 615 files, 369104394 bytes, 0 
xferErrs (0 bad files, 0 bad shares, 0 other)

It took 40min for full backup, 11min incr, while yesterday 1h21min ???
The host was on 24/7.

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