i have done some searching around but couldnt find much on it, apart from the stuff i pasted in from the wuftpd website.
I discovered the feature when downloading a load of stuff from mirror.ac.uk, using the unix ncftp client. I thought it was a really cool feature to be implemented. I dont know of any windows client that does such a thing.
The avalaunch xbox dash ftpd also does domething simlar as well i believe, but works in a different way, and is rather propriety i think.
You can get the source for ncftp from here
http://www.ncftpd.com/ncftp/
So it should be fairly easy to see how its implemented. I'm sure the dev team there will have the relevent info as well.