Perhaps I can ellaborate on why one would override the RadDiagramConnection. I noticed that there is no information explaining how to do this, neither in the Telerik documentation or in the forums.
First of all, my reason for overriding the RadDiagramConnection is because I need additional properties on the connection. We have developed a way to execute diagrams containing business logic, and the connection needs properties as IsPaused and IsCanceled to indicate if the diagram can continue to the next shape or not.
If you wish to use a different connection class, you must also inherit from the RadDiagram. The RadDiagram contains a number of methods that will allow you to provide your custom connection class when the diagram mechanism requires a connection: