I built a service in which you can browse html/css buttons and add new ones to the collection. When on a button's detail page you can access to the code, live edit it and see the result when clicking on a preview button. The fork button takes you to the add page while keeping the edited code so you can upload a new one on top of this.
Actually, I'm not sure if the "fork this button" feature is clear ? Would "Duplicate this button" be better ?
Current UI :
EDIT : It seems like it's not that easy to explain what the feature does so let's say we're in Photoshop. You opened a file, made some changes. In this case, the fork button would create a new (unsaved) document from the current by keeping the changes but not altering the original file.