I am not sure I really understand the problem. What has "dragging a notation" to do with creating "lines"?
Assuming that "dragging the notation" means "moving a label to another location", then the following rules apply.
A (selected) label may or may not be moved freely depending on the label's "Placement" property. The available placement options have the following restrictions:
Free: Anywhere
As the name implies, the label may be moved to any location.
SmartFree: Anywhere
The label may be moved to any location. However, by default the label "snaps" to certain positions. To prevent snapping, press and hold CTRL while moving the label.
Internal/Corners/Sandwich/Sides/8 Pos
The label may be placed at discrete positions relative to its node.
Centered/2 Pos/6 Pos/3 Pos Center
The label may be placed at discrete positions relative to its edge.
Center Slider/Side Slider
The label may be placed freely on its edge or next to its edge at a certain distance.
Creating "lines" (or rather "edges" as connections are called) is done by pressing the mouse on a node that is not selected, dragging the mouse to another node, and finally releasing the mouse. If you start edge creation and release the mouse on emtpy space (instead of a node), a bend is created at that location. In that case, edge creation will continue (even if the mouse is not pressed) until you click on a node or you cancel edge creation. Each left-click on empty space will create an additional bend. Each right-click will undo the last bend creation. Typing ESCAPE will cancel edge creation.