Trucking a light
To constrain trucking of any viewport to a single axis, hold down the key. The truck is constrained to the first axis you move while the key is down.
To accelerate trucking, hold down the 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.