Ghost images made by multiple flash exposure. I did it once. I have not had the need to do it again. It was an experiment in using my flash settings. I would have to read the manual to remember the technique. There are many things you try. I forget more than I remember. The things you use everyday get 99% of your images. It is still useful to experiment. Sometimes you do get to use a long lost skill.
But flash photography and I have had a long relationship in which I controlled very little in our image making. I am a reluctant user of flash and prefer available light. So this was my one and only multiple exposure foray. Thanks Dave.