Trucking a light
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.
To truck a light:
The button highlights when it is on.
For a Target Light, both the light and its target move parallel to the view plane, which is perpendicular to the line along which the light is aimed.
For a Free Light, the light moves in the light's local XY plane. The light remains aimed along its local Z axis.
To truck with the middle mouse button:
To constrain trucking to a single axis:
The truck is constrained to the first axis you use.