Stone explains on the Twitter BlogPhase one of project retweet is to show the developer community how it will work from an API perspective as well as a user perspective.

Were still working out the final details, but this sketch gives you the basic idea. Lets say you follow jessverr, biz thats me, and gregpass but you dont follow ev. However, I do follow ev and the birth of his baby boy was so momentous that I retweeted it to all my followers.Imagine that my simple sketch is your Twitter timeline.