Select the button and the command prompt reads:
Click anywhere on an existing arc or circle and the command prompt reads:
Use any of the snaps and input options to indicate the point you wish the to draw the tangential construction line through. You will see a dynamic roll around the arc/cricle as you move the cursor. Simply select the position required either 'by eye' or by snapping to another snap point which defines the geometric constraint. The line three times the width of the current page is drawn on a layer called 'Constructions' which is set to 'blue' by default.