How dose download accelarators work?

For example flashget.It can download different parts simultaneously.I wonder this.How can it achieve this?General agorithm will be enough for me...

How can i learn file size before i start to downoad?

http://www.faqs.org/rfcs/rfc2616.html

if nothing else, do a GET, and read the content lengh header, then request a multipart/byteranges header for the range of bytes to include in the subsequent GET for the part of the file you need.

put each in a seperate thread, and download the parts individually. of course there is a point of demenishing return (max capacity of the downloader / uploader's connection).