A fork is like a schoolyard football game. Tommy brings a ball to school, and gets to be captain of a team every time, picks the best players for himself, because if he doesn’t, he threatens to take his ball away. After a few weeks of this, the other players get annoyed, and everyone chips in to buy a ball for the group. At this point, Tommy has lost all power – he still has his ball, but he’s not team captain any more. He gets left standing against the wall till the very end to teach him a lesson in humility. So he sits out the games for a while in protest, starts a competing game at the other end of the yard with his ball and his rules. All the best players go to the other game, though, and some of the kids make fun of “Tommy’s” game – invitations to give up his game & join the new one seem more like taunts than honest efforts to include him. After a while, he realises that playing football with others is more fun, and more important, than playing with his football.
My only comment here is that Tommy in this situation represents a master trunk and the others are the fork. There’s an additional football analogy here, that of Tommy wanting to be captain and bringing his own ball to the schoolyard and trying to get the others to play his game. If they do, we admire the power of the open-source ecosphere. If they don’t, it’s another branch that’s shriveled and died.
In this sense, open-source forking has made it into Urban Dictionary.