This is normally overridden for mouse-up tools. An override of this method usually does nothing when DiagramTool.Active is false, except for calling DiagramTool.StopTool.

Tools normally stop upon a mouse up, by calling DiagramTool.StopTool. If you want to handle multiple mouse down-up gestures in one tool activation, you will need to override this method to only stop the tool when you want.

Reference