I believe the specific code has to do with "fnid harvesting". See the comments here: http://github.com/nex3/arc/tree/master/srv.arc line 330 or so.

The general idea is described here: http://dresese.thehyatts.net/archives/000253.html

pg's original patent on this scheme: http://www.freepatentsonline.com/6205469.html