First the scene is composed on the ground glass to fit either the upper or lower half of the ground glass. The L-shaped darkslide is inserted after you remove the normal dark slide before the shutter is opened. Slide the L-shaped dark slide in to cover the opposite half that was composed on the ground glass. Snap the photo then remove the L-shaped dark slide and insert the normal full sized darkslide. The tricky part is remembering which half has been exposed.
I took it out today to try it and got a couple of photos. The best one here. Contact printed on a half sheet of 8x10 Ilford MGIV Deluxe RC paper.
Here is an 8x10 of essentially the same scene for reference.