On a hunch I dusted off my copy of "Star Wars scrapbook" by Stephen Sansweet, and found what I was remembering. It was a replica of the "official Star Wars fanclub newsletter" #1 from 1978.
Transcribed from the book by me below.
The problem of getting the Light Sabers to work fell upon
Special Production and Mechanical Effects Supervisor
John Stears.
(Snip)
The 'blade' part of the Light Saber is actually a four-sided
blade attached to a small motor in the handle. The motor
is used to quickly rotate the blade.
Two sides of the blade are coated with a highly reflective
material similar to the material used on motion picture
screens. One of those two sides is painted four inches
higher than the other side. When spinning, this gave the
blade its flashing effect.
A device was made to lock a light source onto the camera,
allowing the camera both to move freely and to be aligned
with the light source and the blade's reflection.
The film was then taken to a special effects house and
rotoscoped, an animation process involving the frame-
by-frame drawing of previously photographed live action.
this animation process added brightness, flash, and
color to the Light Saber blades.