A link is comprised of two or several points: a starting point, a final point and possibly intermediate points. Once the link has been selected, the selection squares appear at each point forming the link.
When you click one of these squares, you can change the location of the corresponding intermediate point by dragging the mouse while holding the left button, then by releasing the button when the desired position has been reached.
However, there is a restriction relative to the final points of the link: the new location of the point must always be inside the attached object. Otherwise, this location is invalidated.When you click a link (outside of the selection squares) and you drag the mouse while holding the left button, the link is broken and an intermediate point is created where the button was released.
As previously mentioned, all actions on links can be canceled at any moment by pressing the Esc key.