) The very bare bones way it's done is like this -- this is not a full account of how. You basically paint a white bar onto each frame of the shot where you want the saber to go. Using some fancy effects (and After Effects or Premiere seem to have all the tools required to do it) you then create the "outer glow" of the saber and apply it to the white bars you've painted all over your frames.) Hey presto, lightsaber.