Moves the spotlight (and its target) parallel to the view plane.
This button replaces the Pan button when a spotlight viewport is active.
For a target spotlight, dragging the mouse moves the locked spotlight and target parallel to the spotlight view. For a free spotlight, dragging the mouse moves the spotlight along its X/Y axes.
To constrain trucking of any viewport to a single axis, hold down the SHIFT key. The truck is constrained to the first axis you move while the SHIFT key is down.
To accelerate trucking, hold down the CTRL key.
The camera or spotlight and its target move parallel to the view plane, which is perpendicular to the camera's line of sight.