During Lukes lightsaber battle with Darth Vader,Vader falls backward off a ledge and Luke looks over the edge to see where Vader has gone and then walks to the left. just as Lukes lighted lightsaber is hidden from view you hear the sound of it being turned off,and as it passes into view again the "light-blade" is gone.

If you look carfully you can briefly see the tip of the light blade come into view as the hidden stagehand (the one who removed the blade when it was briefly out view)is handling it.