In After Effects, I'm creating lightsabers. Now, when the lightsaber say, shuts off, how can I make the layer "shut off" for that part of the video, then have it come in again?

Not sure if this helps, but you can keyframe the opacity to 0%.

Yeah, what Glenn said. Make sure that you keyframe the last "on" frame to 100% and the first "off" frame to 0%. If you just keyframe from the first "on" frame at 100% to the first "off" frame at 0%, the saber will slowly fade out and in.

If you want to see something with sabers done this way, right click on the following link, save it locally, and watch from your hard drive. The sabers are in the 2nd half.

Cheers,
Bill

http://www.wrgardner.com/film/KidWars/KW.640x360.mov