New Diagram By Selection
Select the elements you want to separate to a new diagram and click on Create New Diagram by Selection in the context bar. That's it. The new diagram with selected elements will be created in the current project. It will show in a new window where you may continue with diagramming. The elements in the new diagram will share their model with the elements in the original elements. The original diagram will stay intact after this operation. The selected element will not be deleted from it.