A link widget allows adjustment of the curvature for curved-style links (the default link style).
Drag the widget (indicated) to adjust the link's curvature. Movement allows both for altering the centre of arc of the curve (the 'x' point) or adjusting the bezier of S-shaped curved links.
Link attachment at the source and destination ends are controlled separately via draggable link anchors.
See also—notes linking to here: