When you take a picture, you usually photograph one image per frame of film. But in a multi-flash photograph, several exposures are made on a single negative. Multi-flash photographs are taken in a darkened room with the camera’s shutter wide open. The film is exposed only when the stroboscope flashes.
The rate of the flash determines the number of pictures on the negative: from a few, separate images (like Doc’s photographs of high-speed divers), to may overlapping ones (like his photographs of golf and tennis swings).