It's what Olympus cameras call Live Composite; it is useful for many kinds of photography such as lightning, fireworks, etc. It makes one image but it's a long exposure that collects individual sub-exposures so that the light doesn't over expose, using an electronic shutter to basically make multiple exposures all stacked and blended into a single image file. This was 30 seconds long and taking six second exposures so that each time the lightning flashed, it only records pixels brighter than those in the underlying frames. Hard to explain unless you understand how to blend layers in Photoshop in Lighten mode, then it all makes sense.