Edit Bar controls: none.
This tool allows you to pick points on entities so that the newly created entity is tangential to the picked entity.
This lock mode automatically switches off after one pick.
For example, to create a line which is tangential to two existing circles, start the Draw>Multiple Lines and Arcs tool in 2 Pt. Line mode, activate the tangent lock mode, pick on the first circle, re-activate the tangent lock mode and pick on the second circle.
Note that there are two possible tangent points on each circle in this example, so pick closest to the one you want.
Related topics: Snap Modes and Lock Modifiers