This new major version of rstWeb now supports the enhanced Rhetorical Structure Theory formalism, eRST. This formalism allows for annotation of secondary edges, super-imposed on the primary RST tree, which can conflict with the RST tree and break tree structure, including multiple edges between nodes. In practice, with secondary edges turned on, users can hold ctrl while dragging to add secondary edges.
For more on the constraints and theory behind eRST, see the eRST website