Codechef Rating System

Can anybody explain me how exactly the rating of any user is calculated ?

I did't get it from this link also

http://blog.codechef.com/2010/12/27/a-surprise-in-store-this-new-year/

Please Explain us with some example. I will really appreciate your effort.

and Please explain me some think about ELO seed based rating system.

I really want to know about it.

thanks