Every time you make an adjustment to the lightsaber, you create a keyframe. EffectsLab automatically interpolates the frames in between.
So, if you use the rotate tool on frame 10, then don't use it again until frame 43, EffectsLab will automatically rotate the lightsaber evenly between those two keyframes, from frame 10 to frame 43. The trick is to use the same tools to move the blade each frame, so you don't introduce unnecessary keyframes. Go back into the lightsaber effect and delete any rotation keyframes you have created. Then you may need to reposition the blade on those frames, but I'm pretty sure that will be the quickest way to fix it.
Fore more information, take a look at the knowledgebase article, "What is Tweening, and how does it work?"