The effect you speak of is a rail-guided mirror within the tube. Point the saber down, and the mirror slides down, allowing the light to "grow" in the tube. To retract, point the saber up, shake the mirror loose, and voila! Lights out.
One word comes to mind: Meh...!
hmm... if the tip holds a strong but small electro magnet on battery with a wireless switch build in on the AV switch of the saber... turn saber on - magnet goes on.. point down... mirror slides up and sticks. Point up and turn saber off.. magnet shuts down immediatly while saber first play down sound and then shuts off the LED.
Shaking the mirror loose from a weak magnet will not hold up in a fight...