Double click on a link to add a junction node as a label of that link. You can then draw new links or reconnect existing links to or from that junction node.
You can also select and copy or delete such junction nodes. To make them easier to select, they are actually about twice as large as the black circle that you see. A junction node is not deletable unless there are no links connected with it.
You can see how junction nodes are represented in the model. This is updated automatically after each transaction.